Is this client in a seperate network?
Is there a firewall somewhere in between? Windows firewall?
From the master, in the NBU GUI, can you connect to this client?
Can you telnet the netbackup master / media server on the netbackup ports 1556 and 13724 (i think)?
13/42 is often caused by a timeout somewhere along the line.
The usually solutions are defragmentation (faster reading of the files), increasing the client read timeout, fix any issue with VSS system. If there are a firewall between the machines decreasing the TCP keepalive time to 5 minutes or less is a good idea.
But without any logs or job details it is impossible to be more specific.
This entire environment is so old that nothing is supported anymore - OS vendors and Veritas.
You need to get OS admins as well as network admins involved to health-check NIC drivers and settings on the servers as well as the network switch.
Mismatched switch port and NIC settings can cause this - check that everything is configured the same way - fixed speed or Auto-negotiate.
TCP Chimney settings on W2003 is also known for causing network errors.
NetBackup is the casualty of your aged infrastructure and is merely reporting the errors - not causing it.