Good evening Andreas and welcome back!
Thank you for reaching out to us.
We are sorry to hear about the issues you have been dealing with. Yikes!
I would love to help you with this topic.
I am glad you found a workaround for now. But that also means that I cannot reproduce the issue on my end.
Could you please send me a copy of the latest version of your WordPress theme / child theme via a link to a .zip file link (stored Dropbox or Google Drive) so that I can run some tests on my end ? I recommend a private reply for that purpose.
Please ensure you are using the latest files as found on your actual website.
This way I will get access to any updates or customizations you might have made.
Additionally, when you say simple layout, could you confirm to which setting in Events -> Settings -> Display (skeleton, full, tribe).
Finally, I would strongly encourage you to upgrade your PHP version to at least 5.4.x +
In fact WordPress recommends 5.6+
This ensures that all of your site runs smoothly.
Have a good weekend,
Geoff B.