LOL! I heard my name...
This is your problem:
Client is Server 2000 running NBU client 6.5.4
Not so much with NBU version, but the Windows software. I remember MANY years ago that there were recommendations regarding MS hotfixes for the TCP stack on W2000.
With W2000 out of MS support for so many years, you have little to no chance of getting any MS updates.
You need to let the server owner know that the issue is with his/her antiquated OS and that there is nothing that can be done from NBU point of view to fix it.
We have had customers with similar situations over the years.
To try and get a backup through, they have done the following:
Ensure checkpoint is enabled in policy attributes - lower the checkpoint interval based on how long the backup runs before failing.
Break up the Backup Selection into smaller chuncks and Allow Multiple data streams.
Limit concurrent jobs to 2.
Keep on resuming Incomplete jobs...
These errors miraculously disappeared when these old servers were replaced with new supported hardware and OS.