Yes, that's it.
Well, this kind of error is really tricky then! :(
While working on a more complex process definition (more than that one I reported), I noticed that the error was thrown almost unpredictably (always after reopening the Eclipse). So when it happens, I revert it to a non-error version from Eclipse local history or the git commits log, then kept on modeling. In my understanding, the XML was already "dirty" after too many deletions.
So, I created a fresh new workspace path (without git too) and a new project from scratch, and then a new simple process definition. So the error occurred again (that's what I reported).
* Is there a fingerprint of my env / Eclipse I can build up and send to you?
* Should I also send a chunk of the Eclipse log?
* I can even make a screencast while creating a new process from scratch.
King regards!
P.S.: Yesterday I have to clean up my entire BPMN file manually. It was just a nightmare: 1 error and about 34 warnings!! hahaha It was a mess, with a lot of orphans SequenceFlows and Taks etc.. Now all the warnings and error are gone, and I also deployed it to server :)
My teammates said: "So! All you have to do now is: DO NOT delete a thing! Be sharp, make correct modeling, no mistakes!" hahahaha!! I'm still saving (and commiting) each step and keeping my eye on the BPMN file myself.