this seems to be a bug and I confirmed it is still there on latest snapshot so please file jira for it and we'll try to get it fixed soon.
it turned out to be misplaced boundary event definition in the bpmn2 xml. That's bug that is being fixed but if you'd like to workaround it you can clone the project via git move the boundary event definition into subprocess and then commit and push back to workbench. That will allow you to run that but not edit it via web designer.
Thanks for reporting it!