"The exception is only reproducible with fast-debug build. I can reproduce the issue on XP SP2 with 6uN-b11 fast-debug, but not 6uN-b11."
implies that this is the threading problem
The most possible reason of the problem is accessing Swing
outside Event Dispatch Thread
We ask submitter to make sure that he creates and uses all Swing code
it might be checked with help of SwingUtilities.isEventDispatchThread() method