you might run into same issue as mine.
I mixed the perisistence.xml for JPA2 from jBPM 5.3 with run-time jars from jBPM 5.4.0.CR1. the problem was gone when I changed to use JPA1's peristence.xml.
I did not try JPA2, but form the https://community.jboss.org/thread/195386?tstart=0. It should work as well.