Hi Hans-Gerd!
Thanks for coming back to us! 🙂
I could not reproduce the issue you are having, so it could be a theme or plugin conflict.
So could you please go through the complete steps in this guide > https://theeventscalendar.com/knowledgebase/testing-for-conflicts/ and let me know what you can find in the process?
I would first try switching to a default theme like twentyfifteen and see if you still get the error. Then, if you still get it, move on with the testing for conflicts guide as it is the best way to spot the source of the issue.
Let me know how it goes and if you have other questions.
Best!
Victor