Did you get the message in the status bar saying that the "Purchase requisition is changed" When you delete the attachment from the said path and save the purchase requisition.
Check the below SAP notes and see whether they are helpful.
825348 - Authorizations for changes in the attachment list
491271 - Authorizations for generic object services
Which version of SAP system you are into?
Try to perform the same activity with another user id and check whether it works with that id. Check and let us know your observations.
