You will be able to track down by breaking the backup selection like c: d: or in job detailed status it will show you where you are getting the 71error code , set verbose log to 5 to track down completly.
Hi
You recently did any patch update or any changes in master as well media server side if so check the compatibility list on both sides IBM and symantec, if not Pls check with density type of drive .
Are you faceing this issue for all STU or on...