Meeting without participant not possible

Hi, I’m using 6.0.24928.0.
Since some time I have the following problem: I use two CalDAV-account. One works fine. The second one seems to work fine too, but when I create a meeting (without setting participants), this meeting is shown as “read-only” in other apps on my smartphone like WeekCal.
This only happes for one of the two CalDAV-Accounts. What can be the problem here?
The only difference between a working meeting appointment and a not working on is the “participants” - however I don’t know, why these two are different. With participant it does NOT work.
Thank You!