I'm sure the VM devs are aware of this. They have done some work on this feature in V2.0.7 and you still get the 500 error, but I feel it still needs more tweaking. Maybe they fixed it in V2.0.7a, which I am going to test today. At the moment you have to physically go into the database (phpMyAdmin) to delete the users in the waiting list table. And for most people, they wouldn't know how to do that. I'm hoping they'll do more work on this in the next few versions.