Sounds nasty, doesn't it?
Worse, there's little clue, even with --trace on, as to why it failed.
In my case, however, I poked around a bit and noticed that my "test/test_helper.rb" file was missing. (Long story as to why.) Restoring it fixed the "Errors running test:units!" problem.
Hope that helps someone else out there ('cause I couldn't find a good explanation online)...