I got the following error:
Exception: org.hibernate.HibernateException: instance not of expected entity type: org.jbpm.graph.action.MailAction is not a: org.jbpm.graph.def.Action
when I tried to deploy a process definition with
<task-node name='start toothpick line'>
<mail to='xxx@mycompany.com' subject='Test01' text='Testing' />
<assignment actor-id='xxx' />
</task-node>