When you cancel a problem task using the cancel UI action a popup will show with the fields you need to fill in. These fields can be incorrect and not contain all the mandatory fields required to actually close the problem task.
Additional mandatory fields were added or the view 'PTASK_Cancel_Dialog_View' was modified.
Go to any problem task record and go to configure->form layout. Make sure the 'PTASK Cancel Dialog View' view at a minimum contains all mandatory fields required to cancel the problem task.