unique constraint failed ciexpanded_data.uniquemodelid

During the sync. proces this message appears:

unique constraint failed ciexpanded_data.uniquemodelid

Since a couple of weeks i got a sync issue with an outlook calendar. All items (as far as i can see) do sync however after a while a sync / constraint error is reported.

Reinstalling the app does not solve the issue, also no issues appear with other mailboxes.

Any idea what is causing it or more importantly how i can resolve it. 

Thanks in advance for your support!

Best regards,

Albert