I'm having the same issue. I'm curious how this manifests itself for others in terms of trying to deploy and use processes. In my case, I can commit processes, forms, and images to drools. But drools BPMN designer doesn't seem to pick up the latest information. However, I can create a process instance via the jbpm-console (it does have the correct diagram) but it doesn't bring up the first form (I even tried simply duplicating the rewards approval process that krisv showed). I have no clue if the issues I'm seeing are related to this particular traceback or something else.
At this point, I'm going to install Jboss 7.0.2 and give that a try.