Workflow cancelled without reasons

Hello,

I'm working on an environment with two workflows : 'Leave request' and time 'Clock-In/Out Corrections'

For the both, everyday several tasks ( tasks sent 11 days ago) are cancelled without reasons.

The tasks are always to status 'Sent' but the workitems are 'Cancelled'.

So the task is visible into POWL but any action is possible.

I have planned all standard reports using SWU3 but I not able to identify the job who cancels the tasks.

Do you have an idea ?

Thanks in advance.

David