diff --git a/addons/google_account/models/google_service.py b/addons/google_account/models/google_service.py
index 7905f4b074a..aaf3d865df7 100644
--- a/addons/google_account/models/google_service.py
+++ b/addons/google_account/models/google_service.py
@@ -146,14 +146,14 @@ class GoogleService(models.AbstractModel):
             res.raise_for_status()
             status = res.status_code
 
-            if int(status) in (204, 404):  # Page not found, no response
+            if int(status) == 204:  # Page not found, no response
                 response = False
             else:
                 response = res.json()
 
             try:
-                ask_time = datetime.strptime(res.headers.get('date'), "%a, %d %b %Y %H:%M:%S %Z")
-            except:
+                ask_time = datetime.strptime(res.headers.get('date', ''), "%a, %d %b %Y %H:%M:%S %Z")
+            except ValueError:
                 pass
         except requests.HTTPError as error:
             if error.response.status_code in (204, 404):
diff --git a/addons/google_calendar/models/res_users.py b/addons/google_calendar/models/res_users.py
index d9c08f13fb4..d8693f633e7 100644
--- a/addons/google_calendar/models/res_users.py
+++ b/addons/google_calendar/models/res_users.py
@@ -97,6 +97,14 @@ class User(models.Model):
         self.ensure_one()
         if self.google_synchronization_stopped:
             return False
+
+        # don't attempt to sync when another sync is already in progress, as we wouldn't be
+        # able to commit the transaction anyway (row is locked)
+        self.env.cr.execute("""SELECT id FROM res_users WHERE id = %s FOR NO KEY UPDATE SKIP LOCKED""", [self.id])
+        if not self.env.cr.rowcount:
+            _logger.info("skipping calendar sync, locked user %s", self.login)
+            return False
+
         full_sync = not bool(self.google_calendar_sync_token)
         with google_calendar_token(self) as token:
             try: