Hey Ben,
Thanks for reaching out!
I noticed that you’ve set your calendar page URL slug to calendar, but have also set the single event URL slug to calendar. This could be causing problems and they should be different, so that instead of calendar and calendar, it’s maybe calendar and event, for example.
Try a different single event URL slug, save your changes, and let us know if this helps with things at all.
If it doesn’t, then don’t change the slugs back to calendar and calendar—they should remain different from each other.
And then in this state, proceed with trying all of the steps recommended here ? https://theeventscalendar.com/knowledgebase/fixing-http-404-errors/
Let us know what you find!
George