This message is caused by an exception on a thread different from the executing test thread. All answers so far boil down to this simple explanation. It is a known bug in Visual Studio not to display any sensible information in that case.

Visual Studio’s test runner totally chokes if a thread other than the executing test thread throws an exception: It gets swallowed and there’s no output, no chance to intercept and debug and no nothing except a burned-down smoldering mess that was supposed to be your unit test.