Users not allowed to Delegate tasks


I have a Flexi Task in my workflow, with Create individual tasks for all group members and Allow Delegation fields checked. When an assignee tries to delegate the task, he get this error message on the delegation page: "You are not allowed to delegate this task". The delegation is working if im the assignee (as a site admin) so i tried to give higher permissions (even Full Control) to the users on the task list and the source list, but that not helped. Tried the Set user permission to on the tasks Configure Action page, no luck. Anyone able to help me? 

Thank you very much!

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