OK, I found a post from Tom Baeyens about Job Executor. I understand now that there is no jobs locking but only hibernate optimistic concurrence control using versionning on jobs data.
But still, I do not understand why there would be concurrence (and so sometime collision) in my case. I have only one JobExecutor, so there can be only one thread to acquire a job end execute it, even if I have two process instances (or more) running at the same time. Is it right ? In this case, I don't see what could be the problem. Could you please help me ?