Hi René,
It’s very odd. Unfortunately, I can’t tell you what exactly is causing this issue as it might come from anywhere.
The best thing to do to find from where this issue is coming from is deactivating plugin one by one. First, could you deactivate all plugins that have not been installed with your theme during its setup and all existing custom PHP code snippets please? And then, could you confirm me that the issue is still persisting please?
As you now, missing updates can create conflicts. Please also make sure that you have the latest theme version and that all recommended plugins are up-to-date. Finally, you could also consider clearing all activated caching system as well as your browser cache before investigating any further. If you want to read more on what browser cache is and how to clear it, here is an article that might help you: How to Clear Internet Cache in Every Major Browser.
We’re waiting for your feedback.
Thank you.