Hi David–welcome back to the forums! Sorry for the trouble here with the number events displayed.
That’s very strange and I definitely see what you’re describing. I visit the page and see five events. I click Next Events and see a no events message. Then, I click back and six total events. Not good!
First off, I see you noted having tried:
Deactivated all other plugins? Yes
Switched to a default theme? Yes
Did you happen to notice any different in those conditions? How about when using the default WordPress Twenty Fifteen theme and deactivating all other plugins together–does the issue still pop up then?
One other thing I’d like to have you try after that is heading to Settings > Pernalinks and simply clicking the Save Changes button without changing any settings. This will flush your permalinks which can help address common 404 messages.
Let’s start there and see what we find. 🙂
Cheers!
Geoff