Actually, you do, it seems, have a membership plugin. Buddypress is exactly such a thing and it uses many 'special page' templates as you know. Also, you probably didn't realize that under certain circumstances, the WP Admin Bar will display the notion that the you're viewing one template (what it can see in PF) but in truth, you're seeing a 'special page' template.
This often happens when people view a Blog Page. The Admin Bar will display Blog Page or even Default, but if these are modified, the results do not appear because in truth, what's actually, visibly seen is a special page template that cannot be modified via the Framework. This was my suspicion all along when I said that it might be a WP 404.
I'll gladly write to Simon with your request, but I'm not hopeful if it's a 'special page' being called. Meanwhile, I'd suggest you write the author of the Event plugin and after describing your setup, ask him if a special page is referenced.