diff --git a/backend/app/api/office.py b/backend/app/api/office.py
index ab9213c..fec5700 100644
--- a/backend/app/api/office.py
+++ b/backend/app/api/office.py
@@ -1,7 +1,7 @@
 import io
 import os
 import hashlib
-from datetime import datetime, timezone
+from datetime import datetime, timezone, timedelta
 from pathlib import Path
 
 from flask import request, jsonify, current_app, send_file
@@ -353,9 +353,22 @@ def onlyoffice_config(file_id):
     AppSettings.set(f'oo_callback_{callback_key}', str(file_id))
 
     # Build the config
-    # The URLs must be reachable by OnlyOffice server (not the browser)
-    base_url = request.host_url.rstrip('/')
-    token = request.args.get('token', '') or request.headers.get('Authorization', '').replace('Bearer ', '')
+    # Internal URL for OnlyOffice to reach our backend (Docker network)
+    internal_url = os.environ.get('ONLYOFFICE_INTERNAL_URL', 'http://minicloud:5000')
+
+    # Generate a long-lived token for OnlyOffice file access (24h)
+    from app.api.auth import create_access_token
+    import jwt as pyjwt
+    oo_file_token = pyjwt.encode(
+        {
+            'user_id': user.id,
+            'type': 'access',
+            'exp': datetime.now(timezone.utc) + timedelta(hours=24),
+            'iat': datetime.now(timezone.utc),
+        },
+        current_app.config['JWT_SECRET_KEY'],
+        algorithm='HS256',
+    )
 
     config = {
         'available': True,
@@ -365,11 +378,11 @@ def onlyoffice_config(file_id):
                 'fileType': ext,
                 'key': f'{file_id}_{f.checksum or "0"}_{callback_key[:8]}',
                 'title': f.name,
-                'url': f'{base_url}/api/files/{file_id}/download?token={token}',
+                'url': f'{internal_url}/api/files/{file_id}/download?token={oo_file_token}',
             },
             'documentType': doc_type,
             'editorConfig': {
-                'callbackUrl': f'{base_url}/api/files/onlyoffice-callback?key={callback_key}',
+                'callbackUrl': f'{internal_url}/api/files/onlyoffice-callback?key={callback_key}',
                 'mode': 'edit' if can_write else 'view',
                 'lang': 'de',
                 'user': {