I tested this example with MySQL and the same error happens (though it does not show up with H2).
Again, problem here is that when both the persistence units are configured (as shown in http://community.jboss.org/people/bpmn2user/blog/2011/03/25/jbpm5--persistence-hello-process-example) there is exception while running 'ksession.startProcess("Hello"); '.
It only one persistence unit is used (either process or task), then it does throw any exception.