Maybe it's usefull to mention: I have jBPM 3.0.2
Solved! (or better: workaround...)
If there is a known bug or if anyone has the same problem:
The above exception was thrown, cuz i tried to implemented Koen Aers idea (see http://jboss.org/index.html?module=bb&op=viewtopic&t=78075) - I wanted to create a taskInstance at the node-enter event of the task-node (trough an Action-Handler).
In my action handler i made a "taskInstance.setPooledActors(pooledActors);"
But it seems that there are problems with the Hibernate-Tables: The entries in the tables "jbpm_taskactorpool" and "jbpm_pooledactors" will be only created if the setPooledActors is called from the right context - the AssignmentHandler.
No I've trown away the approach to assign the task in the ActionHandler and moved it right back to the AssignmentHandler and it works fine. :)
jPBP-Team, could you give me some information how the PooledActors are created?