The error occurs because the operation has been canceled manually or a time limit has been set for the backup and that limit has been exceeded. For example, if Enable Automatic Cancellation was selected in the Schedule Options of the Backup Job Properties, the job will be canceled if the run time exceeds the amount of time specified.
Kindly refer the following technote as well.
Title:- How to enable the Automatic Cancellation feature in VERITAS Backup Exec (tm) 9.x for Windows Servers
http://seer.support.veritas.com/docs/255355.htm
As an additional information, kindly refer the following.
Title:- An error message: "Media Mount Failed" is reported when attempting to perform a backup operation.
http://seer.support.veritas.com/docs/240947.htm
We hope this helps. Let us know if the problem still exists and revert back with the error message.