A temporary workaround has been described here:
http://www.eclipse.org/forums/index.php/t/376638/
But this error has to be addressed and is not at the time of this message.
There is also a bug report tracking this on the BPMN2 Editor side:
https://bugs.eclipse.org/bugs/show_bug.cgi?id=389668
This should also be reported on the jBPM side.