I was doing some TDD one day, when Visual Studio crashed. After a few choice words, I booted things back up. Running the tests after starting back up threw up an error window I'd never seen before. It only showed when I used the R# test runner.
It wasn't very helpful, since half the message didn't seem to be showing. I was able to get the error message using the first trick from this article. That error message wasn't much help either.
Fortunately, this project happened to have a build script which also ran the unit tests. Running the build script returned the full error text. That error message showed me the real problem. It turns out the NUnit settings file was empty. That was causing the root element missing error.
Deleting the NUnit30Settings.xml file from the $AppData$\Local\NUnit directory cleared the problem.