Hmmm, the fix works for us on JasperServer 3.0 and PostgreSQL but causes org.hibernate.exception.ConstraintViolationException when the same user executes reports concurrently.
Removing <prop key="maintainInternalUser">PROPAGATION_REQUIRED</prop> when users have already been synced allows concurrent execution.
Will try upgrading to 3.5 to see if it is resolved.