diff --git a/src/lib/api.ts b/src/lib/api.ts
index c08203e..4c6c7ee 100644
--- a/src/lib/api.ts
+++ b/src/lib/api.ts
@@ -215,3 +215,17 @@ export function syncProtocol(protocol: Protocol): Promise<SyncResult> {
 export function pdfUrl(serverProtocolId: number): string {
   return `${serverUrl}${API_PATH}/pdf.php?id=${serverProtocolId}&jwt=${encodeURIComponent(token)}`;
 }
+
+export interface DebugLogUpload {
+  entries: { ts: number; level: string; msg: string }[];
+  appVersion: string;
+  device: string;
+}
+
+/** Debug-Log-Einträge zum Server übertragen (Endpoint applog.php) */
+export function uploadDebugLog(payload: DebugLogUpload): Promise<{ ok: boolean; stored: number }> {
+  return request<{ ok: boolean; stored: number }>('applog.php', {
+    method: 'POST',
+    body: JSON.stringify(payload),
+  });
+}
diff --git a/src/lib/components/ToolDialog.svelte b/src/lib/components/ToolDialog.svelte
index c35bb83..3956481 100644
--- a/src/lib/components/ToolDialog.svelte
+++ b/src/lib/components/ToolDialog.svelte
@@ -1,6 +1,7 @@
 <script lang="ts">
   import type { Tool } from '$lib/tools/types';
   import type { Device, Protocol } from '$lib/types';
+  import { debugLog } from '$lib/debuglog.svelte';
   import { X } from 'lucide-svelte';
 
   let {
@@ -32,6 +33,7 @@
       onclose();
     } catch (e) {
       error = e instanceof Error ? e.message : 'Fehler beim Ausführen';
+      debugLog.add('error', `Tool "${tool.id}":`, e);
     } finally {
       busy = false;
     }
diff --git a/src/lib/debuglog.svelte.ts b/src/lib/debuglog.svelte.ts
new file mode 100644
index 0000000..89c1edd
--- /dev/null
+++ b/src/lib/debuglog.svelte.ts
@@ -0,0 +1,159 @@
+/**
+ * Debug-Log (Svelte 5 Runes).
+ *
+ * Fängt globale Fehler (window.error / unhandledrejection) und alle
+ * console.*-Ausgaben in einem Ringpuffer ab. So lässt sich nachvollziehen,
+ * warum z.B. ein Scan oder die Synchronisierung scheitert — ohne Kabel,
+ * ohne Logcat.
+ *
+ * Zwei Wege:
+ *  - lokal sichtbar unter Einstellungen → Debug-Log
+ *  - automatischer Upload zum Server (api/applog.php → llx_netdiag_applog),
+ *    damit die Logs auch ohne Gerät auswertbar sind.
+ *
+ * Der Puffer wird in den Preferences gespiegelt und übersteht so einen
+ * App-Neustart (inkl. der Markierung, was schon hochgeladen wurde).
+ */
+
+import { Preferences } from '@capacitor/preferences';
+import { isLoggedIn, uploadDebugLog } from './api';
+import { APP_VERSION } from './updater';
+
+export type LogLevel = 'info' | 'warn' | 'error';
+
+export interface LogEntry {
+  ts: number;
+  level: LogLevel;
+  msg: string;
+  /** true, sobald der Eintrag erfolgreich zum Server übertragen wurde */
+  sent?: boolean;
+}
+
+const MAX_ENTRIES = 200;
+const STORE_KEY = 'netdiag_debuglog';
+const UPLOAD_THROTTLE_MS = 10_000;
+
+/** Beliebigen Wert für die Log-Zeile lesbar machen */
+function fmt(v: unknown): string {
+  if (typeof v === 'string') return v;
+  if (v instanceof Error) return `${v.name}: ${v.message}`;
+  try {
+    return JSON.stringify(v);
+  } catch {
+    return String(v);
+  }
+}
+
+class DebugLog {
+  entries = $state<LogEntry[]>([]);
+  uploadState = $state<'idle' | 'sending' | 'error'>('idle');
+
+  private saveTimer: ReturnType<typeof setTimeout> | null = null;
+  private uploadTimer: ReturnType<typeof setTimeout> | null = null;
+
+  /** Neuen Eintrag anhängen (Ringpuffer) */
+  add(level: LogLevel, ...parts: unknown[]): void {
+    const msg = parts.map(fmt).join(' ').trim();
+    this.entries = [...this.entries.slice(-(MAX_ENTRIES - 1)), { ts: Date.now(), level, msg }];
+    this.scheduleSave();
+    this.scheduleUpload();
+  }
+
+  /** Log leeren */
+  clear(): void {
+    this.entries = [];
+    void Preferences.remove({ key: STORE_KEY });
+  }
+
+  /** Gesamtes Log als Klartext */
+  asText(): string {
+    return this.entries
+      .map((e) => `${new Date(e.ts).toISOString()} [${e.level.toUpperCase()}] ${e.msg}`)
+      .join('\n');
+  }
+
+  /** Persistierten Puffer beim Start laden */
+  async load(): Promise<void> {
+    try {
+      const raw = (await Preferences.get({ key: STORE_KEY })).value;
+      if (raw) this.entries = JSON.parse(raw) as LogEntry[];
+    } catch {
+      /* defektes Log ignorieren */
+    }
+    this.scheduleUpload();
+  }
+
+  /**
+   * Noch nicht übertragene Einträge zum Server schieben.
+   * Best effort — wirft nie und loggt eigene Fehler NICHT (sonst Endlosschleife).
+   */
+  async flush(): Promise<void> {
+    if (this.uploadState === 'sending' || !isLoggedIn()) return;
+    const batch = this.entries.filter((e) => !e.sent).slice(0, 200);
+    if (batch.length === 0) return;
+
+    this.uploadState = 'sending';
+    try {
+      await uploadDebugLog({
+        entries: batch.map((e) => ({ ts: e.ts, level: e.level, msg: e.msg })),
+        appVersion: APP_VERSION,
+        device: typeof navigator !== 'undefined' ? navigator.userAgent.slice(0, 128) : '',
+      });
+      const sent = new Set(batch);
+      this.entries = this.entries.map((e) => (sent.has(e) ? { ...e, sent: true } : e));
+      this.uploadState = 'idle';
+      void Preferences.set({ key: STORE_KEY, value: JSON.stringify(this.entries) });
+    } catch {
+      this.uploadState = 'error';
+    }
+  }
+
+  private scheduleSave(): void {
+    if (this.saveTimer) clearTimeout(this.saveTimer);
+    this.saveTimer = setTimeout(() => {
+      void Preferences.set({ key: STORE_KEY, value: JSON.stringify(this.entries) });
+    }, 1000);
+  }
+
+  /** Upload drosseln: spätestens 10 s nach der ersten neuen Meldung senden */
+  private scheduleUpload(): void {
+    if (this.uploadTimer) return;
+    this.uploadTimer = setTimeout(() => {
+      this.uploadTimer = null;
+      void this.flush();
+    }, UPLOAD_THROTTLE_MS);
+  }
+}
+
+export const debugLog = new DebugLog();
+
+/**
+ * Globale Fehler-Hooks + console.*-Wrapper installieren.
+ * Muss einmalig und möglichst früh aufgerufen werden.
+ */
+let installed = false;
+export function initDebugLog(): void {
+  if (installed || typeof window === 'undefined') return;
+  installed = true;
+
+  void debugLog.load();
+
+  window.addEventListener('error', (ev) => {
+    const ort = ev.filename ? ` (${ev.filename}:${ev.lineno})` : '';
+    debugLog.add('error', (ev.message || 'Fehler') + ort);
+  });
+  window.addEventListener('unhandledrejection', (ev) => {
+    debugLog.add('error', 'Unbehandelt:', ev.reason);
+  });
+
+  // console.* mitschneiden, Originalausgabe bleibt erhalten
+  for (const level of ['log', 'warn', 'error'] as const) {
+    const orig = console[level].bind(console);
+    console[level] = (...args: unknown[]) => {
+      debugLog.add(level === 'log' ? 'info' : level, ...args);
+      orig(...args);
+    };
+  }
+
+  debugLog.add('info', `Debug-Log gestartet — App ${APP_VERSION}`);
+}
diff --git a/src/lib/sync.svelte.ts b/src/lib/sync.svelte.ts
index e701180..cce78b3 100644
--- a/src/lib/sync.svelte.ts
+++ b/src/lib/sync.svelte.ts
@@ -11,6 +11,7 @@ import { Network } from '@capacitor/network';
 import { ApiError, isLoggedIn, syncProtocol } from './api';
 import { getDirtyProtocols, saveProtocol } from './db';
 import { toast } from './toast.svelte';
+import { debugLog } from './debuglog.svelte';
 
 type SyncStatus = 'idle' | 'syncing' | 'offline' | 'error';
 
@@ -77,6 +78,7 @@ class SyncState {
       } catch (e) {
         this.lastError = e instanceof ApiError ? e.message : 'Sync-Fehler';
         this.status = 'error';
+        debugLog.add('error', `Sync (Protokoll ${p.clientUuid}):`, e);
         await this.refreshPending();
         return; // beim nächsten Lauf erneut versuchen
       }
diff --git a/src/routes/+layout.svelte b/src/routes/+layout.svelte
index 1554504..a807577 100644
--- a/src/routes/+layout.svelte
+++ b/src/routes/+layout.svelte
@@ -9,6 +9,7 @@
   import { initDb } from '$lib/db';
   import { registerBackListener, removeBackListener } from '$lib/backButton.svelte';
   import { checkForUpdate, openUpdate, type UpdateInfo } from '$lib/updater';
+  import { initDebugLog } from '$lib/debuglog.svelte';
   import Toast from '$lib/components/Toast.svelte';
 
   let { children } = $props();
@@ -19,6 +20,7 @@
   const HOME = '/auftraege/';
 
   onMount(async () => {
+    initDebugLog(); // zuerst — damit auch Startfehler erfasst werden
     await auth.init();
     await initDb();
     if (auth.loggedIn) await sync.start();
diff --git a/src/routes/debug/+page.svelte b/src/routes/debug/+page.svelte
new file mode 100644
index 0000000..5cdde98
--- /dev/null
+++ b/src/routes/debug/+page.svelte
@@ -0,0 +1,62 @@
+<script lang="ts">
+  import AppHeader from '$lib/components/AppHeader.svelte';
+  import { debugLog } from '$lib/debuglog.svelte';
+
+  const levelColor: Record<string, string> = {
+    info: 'text-zinc-400',
+    warn: 'text-amber-400',
+    error: 'text-red-400',
+  };
+
+  function fmtTime(ts: number): string {
+    const d = new Date(ts);
+    return (
+      d.toLocaleTimeString('de-DE') + '.' + String(d.getMilliseconds()).padStart(3, '0')
+    );
+  }
+
+  // Anzahl noch nicht übertragener Einträge
+  const offen = $derived(debugLog.entries.filter((e) => !e.sent).length);
+
+  const uploadText = $derived(
+    debugLog.uploadState === 'sending'
+      ? 'sendet …'
+      : debugLog.uploadState === 'error'
+        ? 'Upload fehlgeschlagen'
+        : offen > 0
+          ? `${offen} noch nicht übertragen`
+          : 'alle übertragen',
+  );
+</script>
+
+<AppHeader title="Debug-Log" back subtitle={`${debugLog.entries.length} Einträge`} />
+
+<div class="flex items-center gap-2 border-b border-zinc-800 bg-zinc-900 px-3 py-2">
+  <button
+    class="rounded bg-sky-600 px-3 py-1.5 text-sm text-white active:bg-sky-700 disabled:opacity-50"
+    onclick={() => debugLog.flush()}
+    disabled={debugLog.uploadState === 'sending'}
+  >
+    An Server senden
+  </button>
+  <span class="flex-1 text-xs text-zinc-500">{uploadText}</span>
+  <button
+    class="rounded bg-zinc-800 px-3 py-1.5 text-sm active:bg-zinc-700"
+    onclick={() => debugLog.clear()}
+  >
+    Leeren
+  </button>
+</div>
+
+<div class="flex-1 overflow-y-auto p-2 font-mono text-xs">
+  {#if debugLog.entries.length === 0}
+    <p class="p-4 text-center text-zinc-500">Keine Einträge.</p>
+  {:else}
+    {#each [...debugLog.entries].reverse() as e, i (i)}
+      <div class="border-b border-zinc-800/50 py-1">
+        <span class="text-zinc-600">{fmtTime(e.ts)}</span>
+        <span class="break-words {levelColor[e.level]}"> {e.msg}</span>
+      </div>
+    {/each}
+  {/if}
+</div>
diff --git a/src/routes/einstellungen/+page.svelte b/src/routes/einstellungen/+page.svelte
index 95f9b29..64b4cfe 100644
--- a/src/routes/einstellungen/+page.svelte
+++ b/src/routes/einstellungen/+page.svelte
@@ -84,6 +84,20 @@
     </p>
   </section>
 
+  <!-- Diagnose -->
+  <section class="rounded-lg bg-zinc-900 p-4">
+    <h2 class="mb-2 text-sm font-semibold text-zinc-300">Diagnose</h2>
+    <p class="text-xs text-zinc-500">
+      Bei Problemen (Scan/Sync schlägt fehl): Log öffnen und teilen.
+    </p>
+    <a
+      class="mt-3 block rounded bg-zinc-800 px-3 py-2 text-center text-sm active:bg-zinc-700"
+      href="/debug/"
+    >
+      Debug-Log öffnen
+    </a>
+  </section>
+
   <button
     class="mt-2 rounded-lg bg-red-700 py-2.5 font-semibold text-white active:bg-red-800"
     onclick={logout}