Hi,
just make sure that you don't have multiple jbpm installation on your server. For instance, if you installed jbpm in JBoss following user guide and you developed your own application that embed jbpm this can probably cause ClassCastException.
In that scenario you don't need to create process engine any more. You could look it up from JNDI.
Another way is to have only jbpm embedded in your application and not deploy it as separate service.
HTH
Maciej