Local to local backup fails with the message: Network resource cannot be accessed (code: 1513)

I use Cloudberry backup to backup 2 systems on a local lan. System A to System B, and B to A. The backups run on a schedule in the middle of the night when neither system is in active use. Historically, this has worked very well for 6 years, typically 2-5 GB transfer in 2-3 minutes.
(The systems are Windows 10, kept at latest levels. They are ethernet connected through a local router.)

We just added NordVPN and backups are failing. They run for many hours, actually copy about half of the files they intend to backup, then throw the code:1513 when either of the systems become active in the morning.

Things I've tried:
- NordVPN has a split tunneling feature that allows selected bypass tunneling. I added CloudBerry Backup and CBBackupPlan to that list. No joy.
- In Windows Network & Sharing controls, I have loosened the "All" network features to allow public folder sharing for everyone, and to allow sharing without passwords. Again, no joy.

Maybe I've missed something simple / obvious?
Suggestions? (BTW, this help request is going both to CloudBerry and NordVPN)

Thanks in advance!
P.S. MSP 360 is version 7.0.0.648 . NordVPN is verion 6.33.10.0 . Windows is "Windows 10 Home" at level KB4598242 level.