Good evening Chris and welcome back!
Thank you for reaching out to us.
We are sorry to hear about the error 404 page you have been getting.
I would love to help you with this topic.
I would recommend going through the following steps first: https://theeventscalendar.com/knowledgebase/fixing-http-404-errors/
There is a fair chance that this will fix the issue for you.
Finally, please make sure you clear all of your cache especially when it comes down to Comet Cache after each updates of slugs.
Let me know how that goes.
Best regards,
Geoff B.