Marie MM: I did notice the transaction problem if both the persistence units (process and task) are used with Oracle database with the example (http://community.jboss.org/people/bpmn2user/blog/2011/03/25/jbpm5--persistence-hello-process-example). You can also see the problem in your example, if use WSHumanTaskHandler instead of MyAutomaticHumanSimulatorWorkItemHandler (i.e uncomment the following).
//ksession.getWorkItemManager().registerWorkItemHandler("Human Task", new WSHumanTaskHandler());