Good evening Peter and welcome to the Events Calendar Support forum!
Thank you for reaching out to us.
We are sorry to hear about the 404 page.
I would love to help you with this topic.
The first thing I would try is to re-save your permalinks by going to Settings > Permalinks.
As a second 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/
Best regards,
Geoff B.