The error seems to indicate some sort of network trouble, but to pinpoint what it is exactly you need to run few tests. What OS is that? (Windows perhaps)? Are you aware of any changes that have been done that might correspond to time when this started? Are those clients running at the same time? If you have network analyzer do you see any retransmission? etc...

There could be some firewall blocking the way.

You can also check if all network interfaces ar 100 MBps Full Duplex.

Networker is running on Windows 2000 server the clients are Novell 6.5, Windows 2000 2003 and Unix. There have been no changes to our network in last 2 weeks since this started happening no firewall changes. The only changes have been windows updates have been applied to windows servers

According to logs is there possible correlation between Win update and error occurring?

I had the similar problem and did my best to get this rectified.

We used netstat while save -s was running. Netstat showed traffic client attempting to contact server but with no response. Server showed no request being received for the client.

Anyway, the final conclusion came in when we realized that the Cisco Pix firewall upgraded to new firmware was not supporting group based policies, therefore we had to recreate firewall rules for each client IP and the backups are now happening perfectly.