If you open Work tab in the right menu of any Task, you will not see the Complete/Start/Work buttons. The Work tab is completely empty.
This seems to happen regardless of whether the task is created as ad-hoc or as a part of business process and regardless of what roles user has. All our tasklist tests are failing with task form/buttons not being loaded in the Work tab.

EDIT: sorry for confusion, but the issue only seems to appear for ad-hoc tasks, not for tasks generated by business processes.