This bug seems to have existed in the past ( http://wordpress.org/support/topic/404-errors-on-upgrade ) but re-emerged for me recently, with Events Manager 5.2.3 and WordPress 3.4.1.
Basically, everything works fine unless I set the Events page to use a Page which is a child Page of a normal Page. When I do this, the Events page gives a 404 error. Set the Events page to be a main page (no parent) and everything works again.
Interestingly, the plugin is happy with the Locations and Event Category pages being child Pages. No 404 errors there.
Flushing the permalinks didn't help, but I found that deactivating and reactivating the plugin has seemingly fixed it.
My server set-up is as follows:
- Windows Server 2003
- Apache 2.0
- PHP 5.2.17
WordPress is install in the root, not in its own sub-folder.
I hope this info is of use to anyone else who runs into the same problem, and perhaps to Marcus et al in eliminating the bug once and for all.