Hi,
Have you solved this issue?
Do you restart the JBPM server before completing a task?
I got a similar problem here:
https://community.jboss.org/thread/221686?tstart=0
but my setup is I am using JBPM Console Server and Human Task Service that comes with JBPM5.4 Final Full Installer, and I am not restarting the server before completing tasks.
Regards,
Thomas Setiabudi