The following scenario seems to fail:
1) Open a Tx
2) Open a jbpmSession
--> jbpm will create a new Hibernate session in DBPersistenceService, and mark it to be close
3) do jbpm operation(s)
4) close the jbpm session
--> jbpm closes the Hibernate session
5) close the Tx

--> This will fail because of JobSession$DeleteJobsSynchronization which tries to execute operations with the now closed Hibernate session
org.hibernate.SessionException: Session is closed!
at org.hibernate.impl.AbstractSessionImpl.errorIfClosed(AbstractSessionImpl.java:49)
at org.hibernate.impl.SessionImpl.getNamedQuery(SessionImpl.java:1258)
at org.jbpm.db.JobSession$DeleteJobsSynchronization.beforeCompletion(JobSession.java:190)

This is a major issue for jbpm upgrade in JBoss Portal