When trying to run my NUnit tests on the build server, my build turned red with the following error message inside the Test step:
2016-04-28T08:46:53.3339665Z ##[error]Error: Exception System.ArgumentException, Exception thrown executing tests
2016-04-28T08:46:53.3349431Z ##[error]Error: Illegal characters in path.
This problem is caused by a bug in the NUnit 3.0.8 test adapter when used through the NuGet package(and not through the VSIX extension). Luckily a fix is already available, so upgrade to the latest version of the NUnit test adapter and the problem should disappear.