Could you please share the full stacktrace of the exception? In the case of XOR gateways, how are you defining the condition of the branches? How are you modeling your processes? Are you using eclipse, guvnor, or are you ding them manually in plain xml?
As far as I know, complex gateways are not supported by jbpm5. The supported gateway types are: Parallel (AND), Inclusive (OR) and Exclusive (XOR).