I have narrowed this down further...
This time I am using the auto generated simple process and modified it to add a Node via the designer. If I associate an action handler with the Node, I can consistently produce the error. I also noticed that the XML fragment for the generated 'node' element has 'transition' element as the first child followed by the 'action' element. If I switch the order of the two child nodes then the process deploys successfully. This makes sense since the jpdl-3.1.xsd schema for the 'node' element does require a sequence where action precedes other node-content-elements.
It seems that the designer generated XML that violates the schema and the deployer reports HTTP 500 while the underlying problem may be a parse error. I am new to jBPM.. but this is what I observe.
You are correct. This is a bug. I added this JIRA issue.