Some volumes on an NDMP backup fail with a Backup of '/vol/xxx' failed error. (4032900)

The following error messages were in the Binary Log:

Error   2013/10/22 22:00:05   367   Data Plugin   GBKIN-FP01   Backup of '/vol/customer' failed
Error   2013/10/22 22:00:05   367   Data Plugin   GBKIN-FP01   Backup of '/vol/team_win' failed
Error   2013/10/22 22:00:05   367   Data Plugin   GBKIN-FP01   Backup of '/vol/autocad' failed
Error   2013/10/22 22:00:05   367   Data Plugin   GBKIN-FP01   Backup of '/vol/nissantce' failed
Error   2013/10/22 22:00:05   367   Data Plugin   GBKIN-FP01   Backup of '/vol/ng20' failed
Error   2013/10/22 22:00:05   367   Data Plugin   GBKIN-FP01   Backup of '/vol/drwg' failed
Error   2013/10/22 22:00:05   367   Data Plugin   GBKIN-FP01   Backup of '/vol/users' failed

The other volumes within the NDMP backup were backed up successfully.
All the folders reside on the NetApp Filer itself and were not CIFS shares.

Each time the backup was performed, not always the same folders fail on the backup i.e. were random.