Sorry I have not seen this error reported before and have not had it happem during tests. Wonder if it's specific to Windows? The template file used to generate the bpmn2.0jbpm.json is
/stencilsets/bpmn2.0jbpm/stencildata/bpmn2.0jbpm.orig
You may want to try to replace that one with the data of the original bpmn2.0jbpm.json and see if that helps, but again I'm not sure why the generation fails (you cannot turn it off right now btw.). What is the Guvnor version used?