Hi Nitesh,
I have faced this error quite a few time in my environment and most;ly it happens when the load is more on the master or the media server.
running the command nbrbutil -resetMediaServer <mediaserver name> on the affected media server fixes the problem for me.The next step i do if the command doesnt fix the problem is recycle netbackup services on the affected media server.But most of the time nbrbutil will do the trick.
You might want to try the nbrbutil command.The command Resets all nbrb EMM and MDS allocations related to ltid on the media server. Your backup on that media server might be affected though.
If the command works for you.Please look at your backup scheduling and consider load balancing.