Networking Win10 With Win7: The Handle Is Invalid

I'm working on 5 PCs in a workgroup P2P environment. I'm trying to get a Win10 machine to network with a Win7 "server" so a dentistry program can work. The Win10 machine was networking just fine up until about a month ago. Now when that machine tries to run the program it throws an error message specific to the program saying it can't communicate with the server's shared folder & closes the window. All PCs in the office are running the pro versions, Win10 is version 1803. The Win7 machines have homegroup set up but all computers are in the same workgroup also. Win10 doesn't have homegroup - I believe they removed that feature.

When homegroup is enabled, I can get the Win7 machines to communicate amongst themselves OK, but not the Win10 machine. I can see the server's icon under the network list in file explorer, but when I try to access it I get an error message that says, "The handle is invalid". I can ping the Win7 server from Win10 via IP and hostname just fine whether homegroup was enabled or not.

The SOHO network setup appears pretty basic - just a modem/router device from the ISP connected to an unmanaged switch.

I've tried several things:

Disabled homegroup on the server - all machines lost network comms with the server & server icon disappeared from the Win10 machine. I tried changing the workgroup name to reinitialize comms but it didn't work. I re-enabled homegroup for the sake of the Win7 machines.
Ran Complete Internet Repair tool to flush DNS and fix various things.
Disabled firewalls.
Configured static DNS address.
New user profile on the Win10 machine.
System restore on Win10 machine.
Enabled full control of the shared folder on the server to the everyone group.

All that said, I'm just trying to get Windows 10 to play nice with Windows 7 again. Any help is appreciated!