In 2.10.3 (and probably in 2.10.4, because it doesn't look like we're going to have time to backport the fix from 2.11.0-M7), runtime reflection isn't thread-safe: http://docs.scala-lang.org/overviews/reflection/thread-safety.html. Your stack trace is one of the multitude of possible manifestations of the problem.
Bad news is that in 2.10.x there's no workaround for the thread-unsafety issue apart from putting all reflective operations in a synchronized block. Good news is that in 2.11.0 the problem should be gone.