Hey Hans-Gerd,
If the problem goes away in a different theme, then the original problem is indeed a theme issue. There is almost nothing we can do about theme issues, unfortunately. 🙁 I’m sorry about that!
Your best bet is to use the theme that works well, or some other theme that does not cause the conflict. If you really like the theme that causes the conflicts, then you might be able to get some support from that theme developer as to why their theme breaks the events calendar features. They may be able to help.
Best of luck with your project!
George