Sync stopped working with 403 error with iCloud calendar and app-specific password

All of a sudden I'm getting sync errors with a shared calendar, the error log starts with:

Response status code does not indicate success: 403 (Forbidden). : <?xml version='1.0' encoding='UTF-8'?>
<error xmlns='DAV:'>
<valid-sync-token/>
</error>


Action: ReceiveCalendar
Calendar: Familienkalender

XCalendarApp.CalDavProvider.DavAccountRequestException: Response status code does not indicate success: 403 (Forbidden). : <?xml version='1.0' encoding='UTF-8'?>
<error xmlns='DAV:'>
<valid-sync-token/>
</error>

Please let me know if you need the complete error message.

Regards,

KLS