Calendar/Sync/Mac: New dates are »jumping« back to defaults?

After switching from Mac OS Server to Nextcloud my calendar (sync) isn’t really usable. After creating a new date, for example with a duration of two hours, the block is jumping always back to one hour. Sometimes even the title snaps back to the default value, too. Usually i have to wait some minutes and then i’m able to edit the settings with success. It’s annoying :frowning:
I have tried all connection settings, it changes nothing. Further i didn’t find a solution in the web.

Client: Mac, 10.12.6, »Calendar« App