Problem importing to network drive after Catalina upgrade

I have Lightroom Classic running on a MacBook Pro and my external storage drive connected to a Mini on the network. It has worked flawlessly for years in that configuration.

I finally made the switch to Catalina. Now, when I try to import a photo, the import stalls and when I quit the import, I get the error message that the import couldn't complete because the disk is not writable.

However, the disk is writable. If I connect the external drive directly to the MB Pro, it works fine. And I have verified that I can import directly to the internal drive on the MB Pro. Oh, and Lightroom performs perfectly, even though all the image files are on that external network drive that the import error message says is not writable. So it's not really a "writable" issue.

The problem only occurs when I try to import over the network.

This must be a Catalina issue, but I can't figure out what's causing it. I've added Lightroom to the Mini and given it Privacy Full Disk Access permissions. I've cleaned up all the permissions. I even wiped the Mini and reinstalled everything.

Any ideas?