CannotUnloadAppDomain Mbunit and CCNET

Question

Our CCNET build system has been running just fine since we set it up. Recently we moved to another room and changed one IP address on our system and we are getting CannotUnloadAppDomainException from MbUnit which is run from NCover. I have checked our SVN logs to make sure there were no changes and there aren't. Has anyone experienced this before or have any ideas what I should be looking for?