Sorry to see this, @Thomas!
I cannot recreate this issue on my own site, so to help investigate this, can you share two pieces of information with us?
1. Share your site’s system information, which you can learn how to do in this article → https://theeventscalendar.com/knowledgebase/sharing-sys-info/
2. Can you clarify your site’s Permalinks settings? These are the settings that are literally in just Settings → Permalinks in your site’s wp-admin. Are they /%postname%/, for example?
Thank you!
George