I started to get errors like this recently, and the whole debugging experiance has gone to the toilet ever since.
Specifically, it looks like it is not able to do break on exception now. And I can't figure out what it the issue. It happens just when I am using TestDriven.Net, and not when I am attach to a different process. Right now it seems to have fixed itself, but it drove me mad.
Googling doesn't find anything useful :-(