diff --git a/src/carddav_client.py b/src/carddav_client.py
index 81fcc0d..ae3bb73 100644
--- a/src/carddav_client.py
+++ b/src/carddav_client.py
@@ -15,8 +15,9 @@ antwortet der Server mit 403 valid-sync-token, dann fällt der Client automatisc
 auf einen vollständigen Re-Sync zurück.
 """
 import logging
+import os
+from urllib.parse import urljoin, urlparse
 from xml.etree import ElementTree as ET
-from urllib.parse import urljoin
 
 import requests
 
@@ -35,10 +36,11 @@ class SyncTokenInvalid(Exception):
 
 
 class CardDAVClient:
-    def __init__(self, base_url: str, username: str, password: str, timeout: int = 30):
+    def __init__(self, base_url: str, username: str, password: str, timeout: int = 30, account_name: str = ""):
         self.base_url = base_url
         self.auth = (username, password)
         self.timeout = timeout
+        self.account_name = account_name
         self.session = requests.Session()
 
     def _request(self, method: str, url: str, body: str, depth: str = "0"):
@@ -49,6 +51,7 @@ class CardDAVClient:
         if resp.status_code == 403 and "valid-sync-token" in resp.text:
             raise SyncTokenInvalid("sync-token vom Server abgelehnt")
         resp.raise_for_status()
+        self._dump_debug(method, url, resp)
         return ET.fromstring(resp.content)
 
     def discover_principal(self) -> str:
@@ -148,3 +151,16 @@ class CardDAVClient:
         if href.startswith("http"):
             return href
         return urljoin(self.base_url, href)
+
+    def _dump_debug(self, method: str, url: str, resp: requests.Response):
+        dump_dir = f"/app/debug/carddav/{self.account_name}" if self.account_name else "/app/debug/carddav"
+        try:
+            os.makedirs(dump_dir, exist_ok=True)
+            path = urlparse(url).path.rstrip("/").replace("/", "_") or "root"
+            filename = f"{method}_{path}.xml"
+            filepath = os.path.join(dump_dir, filename)
+            with open(filepath, "w", encoding="utf-8") as f:
+                f.write(resp.text)
+            logger.debug("CardDAV-Debug-Dump: %s", filepath)
+        except Exception:
+            logger.warning("Debug-Dump fehlgeschlagen", exc_info=True)
diff --git a/src/config.py b/src/config.py
index 4bef023..fab57cf 100644
--- a/src/config.py
+++ b/src/config.py
@@ -1,7 +1,8 @@
 # Copyright (c) 2026 Stefan Koelle (https://stefankoelle.de)
 # Licensed under the MIT License. See LICENSE file in project root for details.
-import os
 import json
+import os
+
 from dotenv import load_dotenv
 
 load_dotenv()
diff --git a/src/sync.py b/src/sync.py
index 675f6d4..678028d 100644
--- a/src/sync.py
+++ b/src/sync.py
@@ -19,7 +19,7 @@ logger = logging.getLogger("sync")
 
 
 def sync_account(conn, account, href_to_uid_cache: dict):
-    client = CardDAVClient(ICLOUD_BASE_URL, account.apple_email, account.apple_app_password)
+    client = CardDAVClient(ICLOUD_BASE_URL, account.apple_email, account.apple_app_password, account_name=account.name)
     collection_url = client.discover_collection()
 
     stored_token = db.get_sync_token(conn, account.name)