I do not see any jbpm related error here... you might have more luck in the JBoss AS forum I think. Not that it is of no interest to jBPM since it might be caused by the ejb timers.
And yes, it might be related to the config. I unfirtunately have no experience with ejb-timers in jBPM
I dont think that EJB timers should use XA data sources. I don't know if you have it configured that way or the exception is occurring because of something else, but wanted to suggest this to you.
While this doesn't give you a definitive answer on the datasource type of EJB timers, if does give you some XA configuration advice for JBPM.