sorry for not getting back to you earlier.
Unfortunately, in some cases, the Google authentication doesn't work properly. In order to keep the sync service secure, I haven't been able to find a good way around this yet. So instead of using your Google account to register with the sync service, please use a Deliveries account instead. You can also convert your existing Google account into a Deliveries account by using the password reset function: https://deliveries.orrs.de/resetpw/
Make sure to login with that account on all your devices afterwards.
Sorry for the inconvenience!