Calendar: Can't delete reoccuring events (CalDAV)

one of our employees created a reoccuring event in her calDAV calendar and invited other participants.
If she tries to delete one of these reoccuring events (not all of them), it just pops back up after one second.
We also deleted all the other participants and tried to delete the event again, it reappeared with all the participants the previously invited participants.

We already installed the current version of the emClient and re-integrated her CalDAV calendar.