VM backup failed with error code 6

Error bpbrm(pid=33007) from client XXXXX: ERR - Error opening the snapshot disks using given transport mode: Status 23
Critical bpbrm(pid=33007) from client XXXXX: FTL - cleanup() failed, status 6   

the backup failed to back up the requested files(6)

Netbackup server - 7.6.0.1

ESXi version 5.0.0