Hi Franco80,
Not sure how this will work on your environment, but for me, since I use jbpm-console to start my processes this is what I do as a workaround:
After a server restart, make sure that the first thing that I do is start or execute a new process instance where the process will create a human task after it is created. Usually I execute the Evaluation process sample. After that the Human Task WorkItemHandler will be ready to listen to any task completed events emitted by human task service.
Regards,
Thomas Setiabudi