Looks like that tape has developed a fault somewhere, which has been detected by the drive so it's no longer using that part of the tape.
While the backup was still successful I sure as hell wouldn't want to keep using that one long term, so you best bet is probably to replace the tape. Consider how long all of the tapes have been in use, and if they have been in use for a long time it might be safest to consider replacing all of them, to avoid problems with any of the other over time. You might also want to check out the Media Report, and see if any of the other tapes have reported problems, to help make that assessment.
Keith