I have not seen this before. There is no reason the bpmn2.0jbpm.json file should abruptly stop like in your pasted code. When you open the designer, this file get's generate from a default template and workitem configurations present in guvnor (which I assume you have none).
Can you try opening the designer in browser outside of guvnor (http://localhost:8080/designer/editor?profile=jbpm&uuid=12345) and hook up a javascript debugger to see what you get?