1.5.5 ThreadDeath UncaughtExceptionHandler mystery

We just upgraded from 1.4.1 to 1.5.5.

Now we see these super-un-helpful errors in the error log every few minutes.

How can we get the root stack trace logged or possibly determine what is causing this???

Exception: java.lang.ThreadDeath thrown from the UncaughtExceptionHandler in thread "Thread-28"

Never got these errors previously prior to upgrade

--verbose doesn't add anything more...

Running logstash with --debug produces a huge log file but without the error message. The file does not contain ThreadDeath or UncaughtExceptionHandler.
The exception error message gets only printed to logstash.err without any further information.