diff --git a/app/admin/page.tsx b/app/admin/page.tsx
index 1362184..bd7c413 100644
--- a/app/admin/page.tsx
+++ b/app/admin/page.tsx
@@ -317,7 +317,7 @@ export default function AdminDashboard() {
       // Auto-seleziona il font appena caricato
       if (data.name) setPortal(p => ({ ...p, fontFamily: data.name }));
     } catch (err) {
-      showToast(`Network error: ${(err as Error).message}`, 'error');
+      showToast(`Could not reach the server. Check your network connection and try again. Details: ${(err as Error).message}`, 'error');
     } finally {
       setUploadingFont(false);
     }
@@ -336,7 +336,7 @@ export default function AdminDashboard() {
       await refreshFonts();
       if (portal.fontFamily === name) setPortal(p => ({ ...p, fontFamily: '' }));
     } catch (err) {
-      showToast(`Network error: ${(err as Error).message}`, 'error');
+      showToast(`Could not reach the server. Check your network connection and try again. Details: ${(err as Error).message}`, 'error');
     }
   };
 
@@ -669,7 +669,7 @@ export default function AdminDashboard() {
       showToast(`Restore completed: ${data.restored?.cards ?? 0} cards, ${data.restored?.portals ?? 0} portals. Reloading…`);
       setTimeout(() => window.location.reload(), 1200);
     } catch (err) {
-      showToast(`Network error: ${(err as Error).message}`, 'error');
+      showToast(`Could not reach the server. Check your network connection and try again. Details: ${(err as Error).message}`, 'error');
     } finally {
       setRestoring(false);
     }
@@ -707,7 +707,7 @@ export default function AdminDashboard() {
       showToast('Factory preset updated.');
       await refreshFactoryPreset();
     } catch (err) {
-      showToast(`Network error: ${(err as Error).message}`, 'error');
+      showToast(`Could not reach the server. Check your network connection and try again. Details: ${(err as Error).message}`, 'error');
     } finally {
       setSavingPreset(false);
     }
@@ -726,7 +726,7 @@ export default function AdminDashboard() {
       showToast(`Factory reset completed: ${data.restored?.cards ?? 0} cards, ${data.restored?.portals ?? 0} portals. Reloading…`);
       setTimeout(() => window.location.reload(), 1200);
     } catch (err) {
-      showToast(`Network error: ${(err as Error).message}`, 'error');
+      showToast(`Could not reach the server. Check your network connection and try again. Details: ${(err as Error).message}`, 'error');
     } finally {
       setFactoryResetting(false);
     }
diff --git a/app/api/admin/backup/route.ts b/app/api/admin/backup/route.ts
index 1deed59..276e438 100644
--- a/app/api/admin/backup/route.ts
+++ b/app/api/admin/backup/route.ts
@@ -16,7 +16,10 @@ function timestamp(): string {
 
 export async function GET() {
   if (!(await checkSystemBin('zip', '-v'))) {
-    return NextResponse.json({ error: "Binario 'zip' non disponibile sul server." }, { status: 503 });
+    return NextResponse.json(
+      { error: "Cannot create the backup: the 'zip' command is not installed on the server. Ask the administrator to install it." },
+      { status: 503 },
+    );
   }
 
   // -r ricorsivo, - stdout, -x esclude pattern relativi al cwd.
diff --git a/app/api/admin/factory-preset/route.ts b/app/api/admin/factory-preset/route.ts
index 9969ef2..61b3636 100644
--- a/app/api/admin/factory-preset/route.ts
+++ b/app/api/admin/factory-preset/route.ts
@@ -31,7 +31,10 @@ export async function GET() {
 // POST — crea il preset zippando lo stato corrente di data/.
 export async function POST() {
   if (!(await checkSystemBin('zip', '-v'))) {
-    return NextResponse.json({ error: "Binario 'zip' non disponibile sul server." }, { status: 503 });
+    return NextResponse.json(
+      { error: "Cannot create the preset: the 'zip' command is not installed on the server. Ask the administrator to install it." },
+      { status: 503 },
+    );
   }
   await mkdir(FACTORY_DIR, { recursive: true });
 
@@ -58,7 +61,10 @@ export async function POST() {
   // 0 = ok, 12 = "nothing to do" (data/ vuoto: accettiamo, sarà uno zip minimale)
   if (exit !== 0 && exit !== 12) {
     try { await unlink(tmpPath); } catch { /* ignore */ }
-    return NextResponse.json({ error: `zip terminato con codice ${exit}` }, { status: 500 });
+    return NextResponse.json(
+      { error: `Failed to create the preset archive: the 'zip' command exited with code ${exit}. Check the server logs for details. The previous preset (if any) has not been modified.` },
+      { status: 500 },
+    );
   }
 
   await rename(tmpPath, PRESET_PATH);
diff --git a/app/api/admin/factory-reset/route.ts b/app/api/admin/factory-reset/route.ts
index b7af1b5..3585715 100644
--- a/app/api/admin/factory-reset/route.ts
+++ b/app/api/admin/factory-reset/route.ts
@@ -11,14 +11,17 @@ const PRESET_PATH = path.join(process.cwd(), 'factory', 'preset.zip');
 
 export async function POST() {
   if (!(await checkSystemBin('unzip', '-v'))) {
-    return NextResponse.json({ error: "Binario 'unzip' non disponibile sul server." }, { status: 503 });
+    return NextResponse.json(
+      { error: "Cannot perform Factory Reset: the 'unzip' command is not installed on the server. Ask the administrator to install it." },
+      { status: 503 },
+    );
   }
 
   try {
     await stat(PRESET_PATH);
   } catch {
     return NextResponse.json(
-      { error: 'Nessun factory preset configurato. Imposta prima lo stato attuale come preset.' },
+      { error: 'No factory preset is configured on this server: factory/preset.zip is missing. Ask a developer to create a preset first (either via "Save as Factory Preset" in admin or by uploading factory/preset.zip manually).' },
       { status: 404 },
     );
   }
diff --git a/app/api/admin/fonts/route.ts b/app/api/admin/fonts/route.ts
index 57f257b..2821ad9 100644
--- a/app/api/admin/fonts/route.ts
+++ b/app/api/admin/fonts/route.ts
@@ -37,24 +37,31 @@ export async function POST(request: Request) {
     const formData = await request.formData();
     const file = formData.get('file') as File | null;
     if (!file) {
-      return NextResponse.json({ error: 'No file received.' }, { status: 400 });
+      return NextResponse.json(
+        { error: 'No font file was received. Choose a font file before clicking Upload.' },
+        { status: 400 },
+      );
     }
 
     const safeName = sanitizeFontName(file.name);
     const ext = path.extname(safeName).toLowerCase();
     if (!ALLOWED_EXT.has(ext)) {
       return NextResponse.json(
-        { error: `Unsupported font extension. Allowed: ${[...ALLOWED_EXT].join(', ')}` },
+        { error: `Font extension "${ext || '(none)'}" is not supported. Allowed formats: ${[...ALLOWED_EXT].join(', ')}.` },
         { status: 400 },
       );
     }
     if (!safeName || safeName === ext) {
-      return NextResponse.json({ error: 'Invalid font filename.' }, { status: 400 });
+      return NextResponse.json(
+        { error: 'Invalid font filename. The file name must contain at least one letter, digit, hyphen or underscore before the extension.' },
+        { status: 400 },
+      );
     }
     if (file.size > UPLOAD_LIMITS.font) {
-      const mb = (UPLOAD_LIMITS.font / (1024 * 1024)).toFixed(0);
+      const limitMB = (UPLOAD_LIMITS.font / (1024 * 1024)).toFixed(0);
+      const actualMB = (file.size / (1024 * 1024)).toFixed(1);
       return NextResponse.json(
-        { error: `Font too large (max ${mb} MB).` },
+        { error: `Font is too large: ${actualMB} MB (limit: ${limitMB} MB). Web fonts are typically under 500 KB — convert the file to WOFF2 (e.g. with a font-tools subsetter) before uploading.` },
         { status: 413 },
       );
     }
@@ -65,14 +72,14 @@ export async function POST(request: Request) {
     const detected = await fileTypeFromBuffer(buffer);
     if (!detected) {
       return NextResponse.json(
-        { error: 'Font content not recognized (unknown format).' },
+        { error: 'Could not identify the font content. The file may be corrupted or not actually a font. Try re-exporting from your font tool.' },
         { status: 400 },
       );
     }
     const allowed = ALLOWED_DETECTED[ext] ?? [];
     if (!allowed.includes(detected.ext)) {
       return NextResponse.json(
-        { error: `Font content does not match extension (${ext} declared, detected ${detected.ext}).` },
+        { error: `Font content does not match its extension: the file looks like ${detected.ext.toUpperCase()} but the extension is "${ext}". Rename the file or re-export it in the declared format.` },
         { status: 400 },
       );
     }
@@ -83,7 +90,10 @@ export async function POST(request: Request) {
     return NextResponse.json({ ok: true, name: safeName }, { status: 201 });
   } catch (error) {
     console.error('Font upload error:', error);
-    return NextResponse.json({ error: 'Failed to upload font.' }, { status: 500 });
+    return NextResponse.json(
+      { error: 'Unexpected error while saving the font. The upload was aborted. Check the server logs for details and try again.' },
+      { status: 500 },
+    );
   }
 }
 
@@ -93,25 +103,37 @@ export async function DELETE(request: Request) {
     const { searchParams } = new URL(request.url);
     const rawName = searchParams.get('name');
     if (!rawName) {
-      return NextResponse.json({ error: 'Missing name parameter.' }, { status: 400 });
+      return NextResponse.json(
+        { error: 'Missing "name" parameter. Specify which font file to delete.' },
+        { status: 400 },
+      );
     }
     const safeName = sanitizeFontName(rawName);
     const ext = path.extname(safeName).toLowerCase();
     if (!ALLOWED_EXT.has(ext) || !safeName) {
-      return NextResponse.json({ error: 'Invalid font name.' }, { status: 400 });
+      return NextResponse.json(
+        { error: `Invalid font name "${rawName}". Expected a basename ending in ${[...ALLOWED_EXT].join(' / ')}.` },
+        { status: 400 },
+      );
     }
     try {
       await unlink(path.join(FONTS_DIR, safeName));
     } catch (err) {
       const e = err as NodeJS.ErrnoException;
       if (e.code === 'ENOENT') {
-        return NextResponse.json({ error: 'Font not found.' }, { status: 404 });
+        return NextResponse.json(
+          { error: `Font "${safeName}" was not found on the server. It may have already been deleted — refresh the page.` },
+          { status: 404 },
+        );
       }
       throw err;
     }
     return NextResponse.json({ ok: true });
   } catch (error) {
     console.error('Font delete error:', error);
-    return NextResponse.json({ error: 'Failed to delete font.' }, { status: 500 });
+    return NextResponse.json(
+      { error: 'Unexpected error while deleting the font. Check the server logs and try again.' },
+      { status: 500 },
+    );
   }
 }
diff --git a/app/api/admin/restore/route.ts b/app/api/admin/restore/route.ts
index 73c6283..9db78ff 100644
--- a/app/api/admin/restore/route.ts
+++ b/app/api/admin/restore/route.ts
@@ -13,7 +13,10 @@ const UPLOAD_STAGING = path.join(PROJECT_ROOT, '.restore-staging');
 
 export async function POST(request: Request) {
   if (!(await checkSystemBin('unzip', '-v'))) {
-    return NextResponse.json({ error: "Binario 'unzip' non disponibile sul server." }, { status: 503 });
+    return NextResponse.json(
+      { error: "Restore is unavailable: the 'unzip' command is not installed on the server. Ask the administrator to install it." },
+      { status: 503 },
+    );
   }
 
   const sessionId = crypto.randomUUID();
@@ -24,7 +27,10 @@ export async function POST(request: Request) {
     const formData = await request.formData();
     const file = formData.get('file') as File | null;
     if (!file) {
-      return NextResponse.json({ error: 'Nessun file ricevuto.' }, { status: 400 });
+      return NextResponse.json(
+        { error: 'No backup file was received. Choose a .zip backup file before clicking Restore.' },
+        { status: 400 },
+      );
     }
 
     await mkdir(sessionDir, { recursive: true });
@@ -41,7 +47,10 @@ export async function POST(request: Request) {
     });
   } catch (error) {
     console.error('Restore error:', error);
-    return NextResponse.json({ error: 'Errore durante il ripristino.' }, { status: 500 });
+    return NextResponse.json(
+      { error: 'Unexpected error during restore. Existing data was not changed. Check the server logs for details and try again.' },
+      { status: 500 },
+    );
   } finally {
     try { await rm(sessionDir, { recursive: true, force: true }); } catch { /* ignore */ }
   }
diff --git a/app/api/cards/route.ts b/app/api/cards/route.ts
index a6902ab..3059911 100644
--- a/app/api/cards/route.ts
+++ b/app/api/cards/route.ts
@@ -20,7 +20,10 @@ export async function POST(request: Request) {
 
     const validation = validateCard(incomingCard);
     if (!validation.valid) {
-      return NextResponse.json({ error: 'Validation failed', errors: validation.errors }, { status: 400 });
+      return NextResponse.json(
+        { error: 'The card could not be saved because some fields are invalid. See the highlighted errors and fix them, then try again.', errors: validation.errors },
+        { status: 400 },
+      );
     }
 
     if (typeof incomingCard.fullContent === 'string') {
@@ -40,7 +43,10 @@ export async function POST(request: Request) {
     revalidatePath('/'); // Force public portal to update instantly
     return NextResponse.json(incomingCard, { status: 200 });
   } catch {
-    return NextResponse.json({ error: 'Failed to save card' }, { status: 500 });
+    return NextResponse.json(
+      { error: 'Unexpected error while saving the card. The card was not saved. Check the server logs for details and try again.' },
+      { status: 500 },
+    );
   }
 }
 
@@ -51,8 +57,11 @@ export async function PUT(request: Request) {
     await saveCards(updatedCards);
     revalidatePath('/'); // Force public portal to update instantly
     return NextResponse.json({ success: true }, { status: 200 });
-  } catch (error) {
-    return NextResponse.json({ error: 'Failed to reorder cards' }, { status: 500 });
+  } catch {
+    return NextResponse.json(
+      { error: 'Unexpected error while saving the new card order. The order was not saved. Reload the page and try again.' },
+      { status: 500 },
+    );
   }
 }
 
@@ -61,7 +70,10 @@ export async function DELETE(request: Request) {
     const { searchParams } = new URL(request.url);
     const id = searchParams.get('id');
     
-    if (!id) return NextResponse.json({ error: 'Card ID required' }, { status: 400 });
+    if (!id) return NextResponse.json(
+      { error: 'Missing "id" parameter: cannot tell which card to delete. Reload the page and retry.' },
+      { status: 400 },
+    );
 
     const cards = await getCards();
     const filteredCards = cards.filter(c => c.id !== id);
@@ -69,7 +81,10 @@ export async function DELETE(request: Request) {
     await saveCards(filteredCards);
     revalidatePath('/'); // Force public portal to update instantly
     return NextResponse.json({ success: true }, { status: 200 });
-  } catch (error) {
-    return NextResponse.json({ error: 'Failed to delete card' }, { status: 500 });
+  } catch {
+    return NextResponse.json(
+      { error: 'Unexpected error while deleting the card. The card was not deleted. Check the server logs and try again.' },
+      { status: 500 },
+    );
   }
 }	
\ No newline at end of file
diff --git a/app/api/portals/route.ts b/app/api/portals/route.ts
index 133234f..309f982 100644
--- a/app/api/portals/route.ts
+++ b/app/api/portals/route.ts
@@ -24,15 +24,21 @@ export async function POST(request: Request) {
 
     const { valid, errors } = validatePortal(incomingPortal);
     if (!valid) {
-      return NextResponse.json({ error: 'Validation failed', errors }, { status: 400 });
+      return NextResponse.json(
+        { error: 'The portal settings could not be saved because some fields are invalid. See the highlighted errors and fix them, then try again.', errors },
+        { status: 400 },
+      );
     }
 
     // themeColor goes into a <style dangerouslySetInnerHTML> in PublicGrid,
     // so reject anything that is not a strict #RRGGBB.
     if (incomingPortal.themeColor !== undefined && !isValidHexColor(incomingPortal.themeColor)) {
       return NextResponse.json(
-        { error: 'Validation failed', errors: [{ field: 'themeColor', message: 'Colore non valido (atteso #RRGGBB)' }] },
-        { status: 400 }
+        {
+          error: 'Invalid theme color: expected a hex code in the form #RRGGBB (e.g. #1e3a8a).',
+          errors: [{ field: 'themeColor', message: 'Theme color must be a hex code like #RRGGBB.' }],
+        },
+        { status: 400 },
       );
     }
 
@@ -51,6 +57,9 @@ export async function POST(request: Request) {
 
     return NextResponse.json(portals[0], { status: 200 });
   } catch {
-    return NextResponse.json({ error: 'Failed to save portal settings' }, { status: 500 });
+    return NextResponse.json(
+      { error: 'Unexpected error while saving the portal settings. The changes were not saved. Check the server logs and try again.' },
+      { status: 500 },
+    );
   }
 }
\ No newline at end of file
diff --git a/app/api/upload/route.ts b/app/api/upload/route.ts
index 2f01d97..3cbd7eb 100644
--- a/app/api/upload/route.ts
+++ b/app/api/upload/route.ts
@@ -44,6 +44,18 @@ function extractExt(name: string): string {
   return name.slice(lastDot + 1).toLowerCase();
 }
 
+// Formatta bytes in MB con 1 decimale per messaggi all'utente (es. "23.4 MB").
+function fmtMB(bytes: number): string {
+  return `${(bytes / (1024 * 1024)).toFixed(1)} MB`;
+}
+
+// Lista leggibile delle estensioni accettate per famiglia, per i messaggi di errore.
+const EXT_BY_FAMILY: Record<Family, string[]> = {
+  image: ['PNG', 'JPG', 'JPEG', 'GIF', 'WEBP'],
+  video: ['MP4', 'M4V', 'WEBM', 'MOV', 'OGV', 'OGG'],
+  pdf:   ['PDF'],
+};
+
 export function normalizeFilename(originalName: string): string {
   const ext = extractExt(originalName);
   const safeExt = /^[a-z0-9]{1,5}$/.test(ext) ? ext : 'bin';
@@ -71,7 +83,10 @@ export async function POST(request: Request) {
     const file = formData.get('file') as File | null;
 
     if (!file) {
-      return NextResponse.json({ error: 'No file received.' }, { status: 400 });
+      return NextResponse.json(
+        { error: 'No file was received. Please choose a file before uploading.' },
+        { status: 400 },
+      );
     }
 
     const ext = extractExt(file.name);
@@ -85,15 +100,14 @@ export async function POST(request: Request) {
     if (ext === 'svg') {
       if (!isLogoContext) {
         return NextResponse.json(
-          { error: 'SVG accettato solo per upload del logo.' },
-          { status: 400 }
+          { error: 'SVG files can only be used for the portal logo. For other images use PNG, JPG, GIF or WEBP.' },
+          { status: 400 },
         );
       }
       if (file.size > MAX_BYTES.image) {
-        const mb = (MAX_BYTES.image / (1024 * 1024)).toFixed(0);
         return NextResponse.json(
-          { error: `File troppo grande (max ${mb} MB per image).` },
-          { status: 413 }
+          { error: `SVG is too large: ${fmtMB(file.size)} (limit: ${fmtMB(MAX_BYTES.image)}). Optimize the SVG (e.g. SVGO) or remove embedded raster images.` },
+          { status: 413 },
         );
       }
       const text = Buffer.from(await file.arrayBuffer()).toString('utf-8');
@@ -116,17 +130,27 @@ export async function POST(request: Request) {
 
     const family = EXT_FAMILY[ext];
     if (!family) {
+      const allFormats = [
+        ...EXT_BY_FAMILY.image,
+        ...EXT_BY_FAMILY.video,
+        ...EXT_BY_FAMILY.pdf,
+        'SVG (logo only)',
+      ].join(', ');
       return NextResponse.json(
-        { error: `Estensione non permessa: .${ext || '(nessuna)'}` },
-        { status: 400 }
+        { error: `File extension ".${ext || '(none)'}" is not allowed. Accepted formats: ${allFormats}.` },
+        { status: 400 },
       );
     }
 
     if (file.size > MAX_BYTES[family]) {
-      const mb = (MAX_BYTES[family] / (1024 * 1024)).toFixed(0);
+      const hint = family === 'pdf'
+        ? ' Compress the PDF (Ghostscript /ebook preset, Adobe "Reduce File Size", or an online PDF compressor) or split it into smaller documents.'
+        : family === 'video'
+          ? ' Re-encode the video at a lower bitrate or resolution before uploading.'
+          : ' Resize or compress the image before uploading (e.g. lower resolution or convert to WEBP).';
       return NextResponse.json(
-        { error: `File troppo grande (max ${mb} MB per ${family}).` },
-        { status: 413 }
+        { error: `File is too large: ${fmtMB(file.size)} (limit for ${family.toUpperCase()}: ${fmtMB(MAX_BYTES[family])}).${hint}` },
+        { status: 413 },
       );
     }
 
@@ -136,15 +160,15 @@ export async function POST(request: Request) {
     const detected = await fileTypeFromBuffer(buffer);
     if (!detected) {
       return NextResponse.json(
-        { error: 'Tipo del file non riconoscibile dal contenuto.' },
-        { status: 400 }
+        { error: `Could not identify the file content. The file may be corrupted, empty, or saved in an unsupported variant. Try re-exporting it as ${EXT_BY_FAMILY[family].join(' / ')}.` },
+        { status: 400 },
       );
     }
     const allowedMimes = ALLOWED_MIMES[ext] ?? [];
     if (!allowedMimes.includes(detected.mime)) {
       return NextResponse.json(
-        { error: `Contenuto del file non corrisponde all'estensione (.${ext} dichiarato, rilevato ${detected.mime}).` },
-        { status: 400 }
+        { error: `File content does not match its extension: the file looks like ${detected.mime} but the extension is ".${ext}". Rename the file to the correct extension or re-export it in the declared format.` },
+        { status: 400 },
       );
     }
 
@@ -180,8 +204,8 @@ export async function POST(request: Request) {
     // Needs transcoding: bail out early if ffmpeg is unavailable.
     if (!(await isFfmpegAvailable())) {
       return NextResponse.json(
-        { error: 'Video richiede ricodifica ma ffmpeg non è disponibile sul server.' },
-        { status: 503 }
+        { error: 'This video uses a codec that browsers cannot play and needs to be transcoded, but ffmpeg is not installed on the server. Upload the video already encoded as H.264 + AAC in an .mp4 container, or ask the administrator to install ffmpeg.' },
+        { status: 503 },
       );
     }
 
@@ -207,6 +231,9 @@ export async function POST(request: Request) {
     if (tmpPath) {
       try { await unlink(tmpPath); } catch {}
     }
-    return NextResponse.json({ error: 'Failed to upload file.' }, { status: 500 });
+    return NextResponse.json(
+      { error: 'Unexpected error while saving the file. The upload was aborted; nothing was changed. Check the server logs for details and try again.' },
+      { status: 500 },
+    );
   }
 }
diff --git a/lib/restore-zip.ts b/lib/restore-zip.ts
index 766d538..4c456dd 100644
--- a/lib/restore-zip.ts
+++ b/lib/restore-zip.ts
@@ -41,7 +41,7 @@ export async function restoreFromZipFile(zipPath: string): Promise<RestoreResult
       return {
         ok: false,
         status: 400,
-        error: 'Impossibile estrarre lo ZIP.',
+        error: 'Could not extract the archive. The file may not be a valid ZIP, or it may be corrupted. Verify the file by opening it locally before retrying.',
         detail: stderr.split('\n').slice(0, 3).join(' '),
       };
     }
@@ -51,18 +51,26 @@ export async function restoreFromZipFile(zipPath: string): Promise<RestoreResult
     try {
       const raw = await readFile(path.join(extractDir, 'cards.txt'), 'utf-8');
       const parsed = JSON.parse(raw || '[]');
-      if (!Array.isArray(parsed)) throw new Error('cards.txt non è un array JSON');
+      if (!Array.isArray(parsed)) throw new Error('cards.txt is not a JSON array');
       cards = parsed.length;
     } catch (e) {
-      return { ok: false, status: 400, error: `Backup non valido: cards.txt assente o malformato (${(e as Error).message}).` };
+      return {
+        ok: false,
+        status: 400,
+        error: `Invalid backup: the archive is missing a valid cards.txt at its root (${(e as Error).message}). Make sure you are uploading a ZIP created by "Save backup (ZIP)", not just any archive.`,
+      };
     }
     try {
       const raw = await readFile(path.join(extractDir, 'portals.txt'), 'utf-8');
       const parsed = JSON.parse(raw || '[]');
-      if (!Array.isArray(parsed)) throw new Error('portals.txt non è un array JSON');
+      if (!Array.isArray(parsed)) throw new Error('portals.txt is not a JSON array');
       portals = parsed.length;
     } catch (e) {
-      return { ok: false, status: 400, error: `Backup non valido: portals.txt assente o malformato (${(e as Error).message}).` };
+      return {
+        ok: false,
+        status: 400,
+        error: `Invalid backup: the archive is missing a valid portals.txt at its root (${(e as Error).message}). Make sure you are uploading a ZIP created by "Save backup (ZIP)", not just any archive.`,
+      };
     }
 
     // Atomic swap: data → data.bak-<ts>, extract → data.
diff --git a/lib/svg-sanitize.ts b/lib/svg-sanitize.ts
index 52b49cd..28b77f5 100644
--- a/lib/svg-sanitize.ts
+++ b/lib/svg-sanitize.ts
@@ -41,12 +41,12 @@ export type SvgSanitizeResult =
 
 export function sanitizeSvg(input: string): SvgSanitizeResult {
   if (typeof input !== 'string' || input.length === 0) {
-    return { ok: false, error: 'Empty SVG content.' };
+    return { ok: false, error: 'The SVG file is empty. Choose a non-empty .svg file.' };
   }
   // Deve essere un documento SVG: opzionalmente un XML declaration, opzionalmente
   // commenti, e poi un tag <svg>.
   if (!/^\s*(<\?xml[^?]*\?>\s*)?(<!--[\s\S]*?-->\s*)*<svg[\s>]/i.test(input)) {
-    return { ok: false, error: 'Not a valid SVG document.' };
+    return { ok: false, error: 'Not a valid SVG document: the file does not start with an <svg> tag. It may be corrupted or be a different format saved with an .svg extension.' };
   }
 
   let out = input;
diff --git a/lib/transcode.ts b/lib/transcode.ts
index 1468f5f..318ae30 100644
--- a/lib/transcode.ts
+++ b/lib/transcode.ts
@@ -317,7 +317,7 @@ async function runJob(job: TranscodeJob): Promise<void> {
     await updateJob(job.id, {
       status: 'failed',
       finishedAt: Date.now(),
-      error: `Output non valido: ${(e as Error).message}`,
+      error: `Transcoding produced an invalid output: ${(e as Error).message}. The video may be corrupted; try re-exporting from your source tool.`,
       pid: undefined,
     });
     await cleanupTmpForJob(job);
diff --git a/lib/validation.ts b/lib/validation.ts
index b76cceb..e623b3d 100644
--- a/lib/validation.ts
+++ b/lib/validation.ts
@@ -36,47 +36,47 @@ export function validateCard(card: Partial<Card>): ValidationResult {
   const errors: ValidationError[] = [];
 
   if (typeof card.title !== 'string' || card.title.trim().length === 0) {
-    errors.push({ field: 'title', message: 'Il titolo è obbligatorio' });
+    errors.push({ field: 'title', message: 'Title is required. Enter a title before saving.' });
   } else if (card.title.length > CARD_LIMITS.title) {
-    errors.push({ field: 'title', message: 'Titolo troppo lungo', limit: CARD_LIMITS.title, actual: card.title.length });
+    errors.push({ field: 'title', message: 'Title is too long', limit: CARD_LIMITS.title, actual: card.title.length });
   }
 
   if (card.shortDescription !== undefined && typeof card.shortDescription !== 'string') {
-    errors.push({ field: 'shortDescription', message: 'Tipo non valido' });
+    errors.push({ field: 'shortDescription', message: 'Invalid type (expected text)' });
   } else if (strLen(card.shortDescription) > CARD_LIMITS.shortDescription) {
-    errors.push({ field: 'shortDescription', message: 'Descrizione breve troppo lunga', limit: CARD_LIMITS.shortDescription, actual: strLen(card.shortDescription) });
+    errors.push({ field: 'shortDescription', message: 'Short description is too long', limit: CARD_LIMITS.shortDescription, actual: strLen(card.shortDescription) });
   }
 
   if (card.fullContent !== undefined && typeof card.fullContent !== 'string') {
-    errors.push({ field: 'fullContent', message: 'Tipo non valido' });
+    errors.push({ field: 'fullContent', message: 'Invalid type (expected text)' });
   } else if (strLen(card.fullContent) > CARD_LIMITS.fullContent) {
-    errors.push({ field: 'fullContent', message: 'Contenuto troppo lungo', limit: CARD_LIMITS.fullContent, actual: strLen(card.fullContent) });
+    errors.push({ field: 'fullContent', message: 'Content is too long', limit: CARD_LIMITS.fullContent, actual: strLen(card.fullContent) });
   }
 
   // Le card External Link richiedono obbligatoriamente l'URL.
   if (card.cardType === 'EXTERNAL_LINK' && (typeof card.actionUrl !== 'string' || card.actionUrl.trim() === '')) {
-    errors.push({ field: 'actionUrl', message: "L'URL è obbligatorio per le card External Link" });
+    errors.push({ field: 'actionUrl', message: 'URL is required for External Link cards. Enter the destination URL before saving.' });
   }
 
   if (card.actionUrl !== undefined && card.actionUrl !== '') {
     if (typeof card.actionUrl !== 'string') {
-      errors.push({ field: 'actionUrl', message: 'Tipo non valido' });
+      errors.push({ field: 'actionUrl', message: 'Invalid type (expected text)' });
     } else if (card.actionUrl.length > CARD_LIMITS.actionUrl) {
-      errors.push({ field: 'actionUrl', message: 'URL troppo lungo', limit: CARD_LIMITS.actionUrl, actual: card.actionUrl.length });
+      errors.push({ field: 'actionUrl', message: 'URL is too long', limit: CARD_LIMITS.actionUrl, actual: card.actionUrl.length });
     } else {
       try {
         const parsed = new URL(card.actionUrl);
         if (!ALLOWED_URL_SCHEMES.has(parsed.protocol)) {
-          errors.push({ field: 'actionUrl', message: `Schema URL non ammesso (${parsed.protocol}). Usa http, https, mailto o tel.` });
+          errors.push({ field: 'actionUrl', message: `URL scheme not allowed (${parsed.protocol}). Allowed schemes: http, https, mailto, tel.` });
         }
       } catch {
-        errors.push({ field: 'actionUrl', message: 'URL non valido' });
+        errors.push({ field: 'actionUrl', message: 'Invalid URL format. Use a complete URL such as https://example.com.' });
       }
     }
   }
 
   if (card.cardType !== undefined && !VALID_CARD_TYPES.includes(card.cardType as CardType)) {
-    errors.push({ field: 'cardType', message: `Tipo card non valido: ${String(card.cardType)}` });
+    errors.push({ field: 'cardType', message: `Invalid card type "${String(card.cardType)}". Pick a type from the dropdown.` });
   }
 
   return { valid: errors.length === 0, errors };
@@ -87,17 +87,17 @@ export function validatePortal(portal: Partial<Portal>): ValidationResult {
 
   if (portal.title !== undefined) {
     if (typeof portal.title !== 'string') {
-      errors.push({ field: 'title', message: 'Tipo non valido' });
+      errors.push({ field: 'title', message: 'Invalid type (expected text)' });
     } else if (portal.title.length > PORTAL_LIMITS.title) {
-      errors.push({ field: 'title', message: 'Titolo portale troppo lungo', limit: PORTAL_LIMITS.title, actual: portal.title.length });
+      errors.push({ field: 'title', message: 'Portal title is too long', limit: PORTAL_LIMITS.title, actual: portal.title.length });
     }
   }
 
   if (portal.welcomeText !== undefined) {
     if (typeof portal.welcomeText !== 'string') {
-      errors.push({ field: 'welcomeText', message: 'Tipo non valido' });
+      errors.push({ field: 'welcomeText', message: 'Invalid type (expected text)' });
     } else if (portal.welcomeText.length > PORTAL_LIMITS.welcomeText) {
-      errors.push({ field: 'welcomeText', message: 'Testo di benvenuto troppo lungo', limit: PORTAL_LIMITS.welcomeText, actual: portal.welcomeText.length });
+      errors.push({ field: 'welcomeText', message: 'Welcome text is too long', limit: PORTAL_LIMITS.welcomeText, actual: portal.welcomeText.length });
     }
   }