steps to reproduce:

• start distro (with deployed invoice showcase)
• login to tasklist as "demo" user
• delegate task "Assign Approver" to another group member (to "John")
• logout

• login again as user "john"
• press button to start work on task "Assign Approver"
• "complete" the task
• logout

• login once again as user "demo"
• press button to start work on task "Assign Approver"
• "complete" the task
• the task has been not completed and is still assigned to user "demo"

-> the task "Assign Approver" should be completed after user "demo" has pushed the button "Complete Task"