Whilst moving over to our new TFS 2012 installation I got the following error when a build tried to run tests
TF900546: An unexpected error occurred while running the RunTests activity: 'Unable to load one or more of the requested types. Retrieve the LoaderExceptions property for more information.'.
This was a new one on me, and nothing of much use on the web other than a basic MSDN page.
Turns out the immediate fix is to just restart the build controller. Initially I did this after switching to the default build process template, and setting it to NOT load any custom activities, but I seems a simple restart would have been enough as once I re-enabled all custom activities it still worked.
As to the root cause I have no idea, one to keep an eye on, especially as I am currently on the RC, lets see what the RTM build does.