Verify the network connection between networker server and the problematic client is normal.

Suggest:

1. Ping/nslookup/rpcinfo
2. Use Hosts files
3. Delete peer information https://community.emc.com/docs/DOC-20085
4. Verify Short and long aliases in client resource in NMC

Which NetWorker version do you use?

When does this error happen (full / incr backup, heavy NW server load, ...)?

Currently we have the same error with multiple clients and from what I heared it could be a common error in v 8.0.

May i suggest you use the NW Health Check Tool (HCT), available from support.

Pls chk the following:

Group inactivity time-out. Ideally it should be set to 0.

Have a look for tcp chimney and if enabled, disable it and try the backup.

change the tcpkeepalive time from default of 2 hours to 15-30 mins.