I noticed yesterday that clicking a single event will return an error 404 page on our site. /events would display correctly, but any /event/event-name style link returned the same error. I was able to fix the problem by changing the single event slug to /event1/. I immediately switched back to /event/ and it was still working.
This afternoon, someone else noticed the single event pages were again giving a 404 error. This time, I went into the plugin settings and just scrolled down to click “update”, making no changes to the slug. This fixed the problem again.
Between the first fix and the second, our site was automatically updated to WordPress 4.1.1, so I’m not sure if the problem was related, but the first bug happened prior to the 4.1.1 update.