Network resource cannot be accessed (code: 1513) when using a VPN
This problem is definitely NOT a Cloudberry problem.
For people who come upon similar problems later: Backups from one system on a LAN to another, and for that matter simple file access from one system on a LAN to another, require the ROUTER to also have VPN software matching the end points. That was the "something simple" I missed. Each of the VPN service providers, NordVPN, ExpressVPN, etc. has software for many different routers.
If you can't install VPN software on your router (for example those routers owned by ISPs), the alternative is to disconnect both ends of a backup process for the time needed.
THANKS David for attempting to help.
... and yes, backup with version 7 of Cloudberry works perfectly when each end of that VPN tunnel is disconnected.