My guess: a race condition which occurs when two threads try to do marshalling at the same time.
True, the code you posted looks like it should be thread-safe. But it's possible that it's running into non-thread-safe situations deep down in the system code which you see at the top of your stack trace.
Or perhaps "JAXBContext.newInstance(CRM_TYPE)" isn't a thread-safe operation and it can return the same object to two different threads. Problems with that theory: I don't know whether that would matter, and the API documentation doesn't mention that as a problem.
Unfortunately that's just speculation, and it isn't clear what you could do to fix it, or how to
test that the fix worked. Perhaps somebody else will have a more useful answer...