OK, this is a total mystery still, I thought you would see that new debug dialog before the check failure happened. I have no idea how that check failure could happen, it's in code that should not even be called by that point.
I think the best thing to do now is this:
I need a volunteer: someone who's experiencing the problem. I'll need a new user account on your computer with admin permissions, and then enable remote desktop so I can access the machine. I'll need to install some special debugging software so I can diagnose the problem on a machine where this is happening. Obviously you need to trust me with your computer... hopefully you do! I'll do my best to clean up everything I've installed when I'm done, they'll all be official Microsoft tools for debugging.
I think that's the only way to get to the bottom of this quickly - the way things are going now it could be days or weeks. Any volunteers? (Sorry it's come to this, I really think it's the final option.)