Maciej, here's the demo application, it's intructions are inside.
Due to lack of time, it does not reproduces the same concurrency error.
But maybe you can find what I'm doing wrong with this.
Of course it's a very basic application and I had to simplify a lot of code.
For example, getting a task service, adding users manually instead of using ldap, a local .bpmn2 file instead of a remote server, etc.
I'm not sending a war file because of the forum attachment size limit.
Thank you very much for helping me, I'm in kind of a tight spot here.