Client backups failing with "machine going down" and "change of preferred address" (4044498)

Description:
During client backup, some of them fail with log error messages:
- machine xxx Changed it's preferred address from xxx.. to xxy..
- Machine xxx has gone down
- Cannot gain network access to client machine
- Failed to start plugin
- Backup Failed

Symptoms:

The message "changed it's preferred address from 10.. to 10..." suggests that the NetVault server or the clients concerned have more than one interface.
Multiple NICs on machines that need to establish a durable connection for streaming data can lead to confusion when the NIC to be used is not specified.
On the machine(s) that possess multiple NIC (or multihomed/several IPs), we need to specify the Preferred Network address that will always be used
and bar the one(s) we won't be using.