diff --git a/radicale/__init__.py b/radicale/__init__.py
index ae53c893..f70e58dc 100644
--- a/radicale/__init__.py
+++ b/radicale/__init__.py
@@ -842,6 +842,4 @@ class Application:
             headers = {"Content-Type": "text/xml; charset=%s" % self.encoding}
             status, xml_answer = xmlutils.report(
                 base_prefix, path, xml_content, collection)
-            if status == client.PRECONDITION_FAILED:
-                return PRECONDITION_FAILED
             return (status, headers, self._write_xml_content(xml_answer))
diff --git a/radicale/storage.py b/radicale/storage.py
index 711e7fc3..f77143ce 100644
--- a/radicale/storage.py
+++ b/radicale/storage.py
@@ -1268,17 +1268,20 @@ class Collection(BaseCollection):
                     flags = LOCKFILE_EXCLUSIVE_LOCK if mode == "w" else 0
                     overlapped = Overlapped()
                     if not lock_file_ex(handle, flags, 0, 1, 0, overlapped):
-                        raise RuntimeError("Locking the storage failed: %s" %
-                                           ctypes.FormatError())
+                        raise RuntimeError("Locking the storage failed "
+                                           "(can be disabled in the config): "
+                                           "%s" % ctypes.FormatError())
                 elif os.name == "posix":
                     _cmd = fcntl.LOCK_EX if mode == "w" else fcntl.LOCK_SH
                     try:
                         fcntl.flock(cls._lock_file.fileno(), _cmd)
                     except OSError as e:
-                        raise RuntimeError("Locking the storage failed: %s" %
-                                           e) from e
+                        raise RuntimeError("Locking the storage failed "
+                                           "(can be disabled in the config): "
+                                           "%s" % e) from e
                 else:
-                    raise RuntimeError("Locking the storage failed: "
+                    raise RuntimeError("Locking the storage failed "
+                                       "(can be disabled in the config): "
                                        "Unsupported operating system")
                 cls._lock_file_locked = True
         try:
diff --git a/radicale/xmlutils.py b/radicale/xmlutils.py
index 248cd8f6..ff91f822 100644
--- a/radicale/xmlutils.py
+++ b/radicale/xmlutils.py
@@ -127,6 +127,13 @@ def _href(base_prefix, href):
     return quote("%s%s" % (base_prefix, href))
 
 
+def _webdav_error(namespace, name):
+    """Generate XML error message."""
+    root = ET.Element(_tag("D", "error"))
+    root.append(ET.Element(_tag(namespace, name)))
+    return root
+
+
 def _date_to_datetime(date_):
     """Transform a date to a UTC datetime.
 
@@ -997,7 +1004,7 @@ def report(base_prefix, path, xml_request, collection):
     """
     multistatus = ET.Element(_tag("D", "multistatus"))
     if xml_request is None:
-        return multistatus
+        return client.MULTI_STATUS, multistatus
     root = xml_request
     if root.tag in (
             _tag("D", "principal-search-property-set"),
@@ -1039,9 +1046,10 @@ def report(base_prefix, path, xml_request, collection):
             sync_token, names = collection.sync(old_sync_token)
         except ValueError as e:
             # Invalid sync token
-            collection.logger.info("Client provided invalid sync token %r: %s",
-                                   old_sync_token, e, exc_info=True)
-            return client.PRECONDITION_FAILED, None
+            collection.logger.warning("Client provided invalid sync token %r: "
+                                      "%s", old_sync_token, e, exc_info=True)
+            return (client.PRECONDITION_FAILED,
+                    _webdav_error("D", "valid-sync-token"))
         hreferences = ("/" + posixpath.join(collection.path, n) for n in names)
         # Append current sync token to response
         sync_token_element = ET.Element(_tag("D", "sync-token"))