Backup on a Linux box progresses as it should, up to about 4.3MB of progress. It hangs there showing no errors.
If canceled, it shows 5MB of progress and no errors.
The cause was that the client was using a Maximum Transmission Unit (MTU) set for jumbo packers: MTU=5000
The other systems were set for standard MTU of 1500.
Setting the MTU on the client having the issues so it matches the other systems and this will resolve the problem.