Error message with CalDav Server (nextcloud) and appointment creation

How do you do?
I am using eM Client version 10.1.4588 (f11e2ab) and cannot create any appointments in the CalDav calendar (nextcloud) I have set up. I get the error message that it is an internal server error.
However, I can create appointments on the same server with other clients from Windows and Android without any problems.
Does anyone have any ideas?