Good evening Jan and welcome back!
Thank you for reaching out to us.
We are sorry to hear about the 404 error messages you have been getting.
I would love to help you with this topic.
Typically, re-saving the permalinks fixes the issue. But if that does not work, I would recommend checking the following steps: https://theeventscalendar.com/knowledgebase/fixing-http-404-errors/
If that still does not work, as a first troubleshooting step, could you please provide us with your complete system information in a private reply using the instructions found in the following link?
https://theeventscalendar.com/knowledgebase/sharing-sys-info/
Let me know how that goes.
Best regards,
Geoff B.