Today I saw my colleagues CalDAV calendar does sync, until a multi day event appeared, after that Ne more events where synced. Is that a possible error source and does it make sense? I will try to replicate that with the Kerio test-Account.

Hey, currently undergoing quite a few changes in our CalDAV integration. Would love to have you test it once ready. Prospective ETA should be a week from now.

I will definitely help testing with our team.