A while ago I posted about seeing the TF900546 error when running unit tests in a previously working TFS 2012 build. The full error being:
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.'.
Well late last week this problem came back with avengeance on a number of builds run on the same build controller/agent(s). Irritatingly I first noticed it after a major refactor of a codebase, so I had plenty of potential root causes as assemblies had been renamed and it was possible they might not be found. However, after a bit of testing there were no obvious candidates as all tests worked fine locally on my development PC, and a new very simple test application showed the same issues. It was defiantly an issue on the build system.
I can still find no good way to debug this error, Stackoverflow mention Fuslogvw and WinDbg, as well as various copy local settings and the like. Again all seems too much as this build was working in the past, just seemed to stop. I tried a couple but got no real information, and the error logs were empty.
In the end I just tried what I did before (as I could think of no better tactic to pin down the true issue). I went into the build controller config, removed the reference to the custom assemblies, saved this settings (causing a controller restart), then put it back (another restart of the controller)
After this my test started working again, with no other changes
Interesting a restart of VM running the build controller did not fix the problem. However this does somewhat chime with comments in the StackOverFlow thread that causing the AppPool in MVC apps to rebuild completely, ignoring any cached assemblies, seems to fix the issue.