All Events and Category pages “not found”

Recently, our Events Calendar plug-in started behaving strangely. First, advanced features, such as repeating events, disappeared from the admin, despite the fact that we have Events Calendar Pro installed and activated.

Then the All Events and category pages started returning a message saying “Not Found”. No other pages/posts on the site appear to be affected.

Individual events can be seen, when you click the ‘view’ link in the admin. On the All Events and category pages, the Events Calendar debugging script shows that events are being returned by the query but they are not displaying.

I’ve reviewed all available posts on the topic. So far, we’ve tried:

Reverting to the WordPress twenty seventeen theme.
De-activating all other plug-ins.
Updating all Events Calendar plug-ins.
Updating WordPress.
Flushing all caches.
Flushing permalinks.
Using default permalinks.
Adding the following snippets:

https://gist.github.com/andrasguseo/b3283cb4503c880a205448dbafc19f7b

https://gist.github.com/elimn/d3483854e8e9399e449e

Nothing has any effect.

I’ve linked to an example category page on a staging version of our site with the twenty seventeen theme and all other plug-ins off. The debugger shows the returned query info but at the bottom of the page you can see the ‘not found’ message.

The page I need help with: [log in to see the link]