I was with this customer today. The problem seems to be related to a large number (300+) of backups that predated NetBackup and weren't cleaned up. Other backups without this issue are backing up fine. The dba is going to try to remove them to see...
About
CERTIFICATIONS
Symantec NetBackup 7.5 for Unix Administrator
SCS Symantec NetBackup 7.5 for Unix Administrator