Hi,
I wanted to update as to how my issue got resolved.
My environment was on a multi-node cluster. When I created a new task (and added to the System Manager role), the new task was visible only from one node. And, whenever the Bulk Task request landed on the node where the task wasn't visible, we saw the "not authorized" error.
In my case, just restarting the nodes resolved the issue. But, I think, clearing the temp folders when restarting the nodes would be a good practice.
Regards,
Piyush.