Hello,
It seems to me like the server can’t process some specific event. You can try to
set another past event time limit, e.g. 0 days disable past event time limit (set to blank) → this will use PROPFIND instead of REPORT, so the server doesn’t need to process the events; but it won’t scale that well, especially if the server doesn’t support Collection Synchronization copy events from the faulty calendar to a new one (maybe this fixes the event) or – of course – contact GMX about the server problem.Have you already asked GMX support about it?