Hi Maciej,
I'm getting the same issue "No active JTA transaction on joinTransaction call", in another case:
if I create a process instance and restart my application or the jboss server, and try to close the porcessInstance (and close complite the current human task),
it works fine for the processinstances newly created (after restarting the app).
do you know if I need to set the local to true (SyncWSHumanTaskHandler) somewhere else.
For the moment I set this variable after loading or creating the StatefulKnowledgeSession.
Thanks for your help.