Yeah, I could narrow down the conflicting event by playing with the past event setting. With past event up to 112 days everything works fine. The event in question is one with accidentally interchanged start and end date. I have no clue, why in spite of this the multi days event was displayed corrently - and moreover could even be synced with CalDAV-sync. As soon as I have corrected start and end time, I am able to sync past events with no limit.
Thanks for feedback. So topic could be marked as “solved”.