Randomly getting status: 21: socket open failed for backups

Hello,

I have been getting errors as follows randomly for both standard and oracle (RMAN) backups on my netbackup master server.

The version of the server is  7.5.0.7 running on RHEL 5.11

The version of the clients are 7.5.0.4  (SAN Clients)

Ther version of the Media servers are 7.5.0.4 (SAN Media).

The logs shows the following, I would like to know if there is any way to start digging for the cause of this issue:

07/05/2015 00:00:37 - Info nbjm (pid=9601) starting backup job (jobid=1635947) for client seprdu01, policy RMAN_SEPRDU01_BD, schedule Default-Application-Backup
07/05/2015 00:00:37 - Info nbjm (pid=9601) requesting STANDARD_RESOURCE resources from RB for backup job (jobid=1635947, request id:{64B718AC-22CA-11E5-8292-439B8376C6B8})
07/05/2015 00:00:37 - requesting resource sesegx2x-hcart3-robot-tld-1
07/05/2015 00:00:37 - requesting resource sesegx10.sagir.qc.NBU_CLIENT.MAXJOBS.seprdu01
07/05/2015 00:00:37 - requesting resource sesegx10.sagir.qc.NBU_POLICY.MAXJOBS.RMAN_SEPRDU01_BD
07/05/2015 00:00:37 - Waiting for scan drive stop HP.ULTRIUM4-SCSI.002, Media server: sesegx27
07/05/2015 00:00:38 - granted resource  sesegx10.sagir.qc.NBU_CLIENT.MAXJOBS.seprdu01
07/05/2015 00:00:38 - granted resource  sesegx10.sagir.qc.NBU_POLICY.MAXJOBS.RMAN_SEPRDU01_BD
07/05/2015 00:00:38 - granted resource  S00830
07/05/2015 00:00:38 - granted resource  HP.ULTRIUM4-SCSI.002
07/05/2015 00:00:38 - granted resource  sesegx27-hcart3-robot-tld-1
07/05/2015 00:00:38 - granted resource  TRANSPORT
07/05/2015 00:00:38 - estimated 0 kbytes needed
07/05/2015 00:00:38 - Info nbjm (pid=9601) started backup (backupid=seprdu01_1436068838) job for client seprdu01, policy RMAN_SEPRDU01_BD, schedule Default-Application-Backup on storage unit sesegx27-hcart3-robot-tld-1
07/05/2015 00:00:39 - started process bpbrm (pid=774)
07/05/2015 00:03:34 - Error bpbrm (pid=774) bpcd on seprdu01 exited with status 21: socket open failed
07/05/2015 00:03:34 - Info bpbkar (pid=0) done. status: 21: socket open failed
07/05/2015 00:03:34 - end writing
socket open failed  (21)