First you are 2 point release behind. LR Classic CC is up to 7.2. Use the Creative Cloud Desktop App to update LR C CC to the current version.
Actually, IF you're both using the same version of LR it sounds more like a like a network drive issue--permissions to be exact. If you can drag the folder to your desktop and then import, your permissions for those files are different than on the network drive. I would check with you network admin to find out if any particular changes were made, say like around November when the import problem began.
I think you may have encountered a bug. If I attempt to "Import from Another Catalog" using LR Classic, and the catalog to be imported is on a network drive, the attempt fails with the same message that you are seeing. Using the same procedure using LR6 works. Oddly, if back in Classic I attempt to import an earlier version catalog from the network drive, that in fact also works (although during the necessary catalog upgrade, it writes the upgraded catalog into the local Pictures folder).
I'll pass the info along to Adobe to see if I can get an answer, but in the meantime it looks as though you're stuck with copying the catalog to a local drive before you can import it into Classic.
PS....it works OK on Windows 10, so I think it's definitely a bug.