You found any solution to this problem?
I've seen this on one occasion, and it happens during startup of the site only after a crash or recycle that allows a new process to be started before the old one dies.
Set "Disable Overlapped Recycle" to true on the application pool to prevent this. It will give the application pool ample time to stop the existing process before a new one is started.
Had this problem too when removing a VisitorGroupCriterion.
Found that the VisitorGroupCriterion was still in use.
Removing the criterion from all visitor groups before removing the criterion implementation solved this.
We recently started getting this error in one of our environments, and after a bit of digging, I can't find out what's causing it. Was hoping someone could point me in the right direction. Thanks!