Hi,
I have found the solution to the issue - a redirect in the htaccess that was created to enable the old logic of using the URL scheme
http://www.domain.com/123 to link to Article 123 in joomla, which is by default not possible in Joomla 5. But the redirect actually worked on everything that had number at the end of the url, which is also the case for many addresses JEvents uses. So by introducing an exception for URLs with /layout/ in the address, now I am able to see the edit window in the modal pop-up for creating and editing events again.
So the case can be closed.
Thanks and best regards
Raimund