I have LR 5 loaded on my Windows machine and my Mac.
I usually do work on the go on my Mac, export the folder I worked on as catalog (without the negatives or previews), and import it into my Windows LR. The folder structure is identical on both machines, except for the very top level folders. This procedure worked until I upgraded to LR 5.7. Now I get the error on my Windows machine: "Lightroom could not import this catalog because of an unknown error." Very descriptive. Thanks Adobe!
So far I have downgraded both machines to LR 5.6, but I still get the same error. Or maybe it's unrelated.
I have also copied the folder from Mac to Win just to make sure there was no corruption and tried the catalog export/import, with same results.
Tried disabling antivirus, still no dice.
Exporting catalog with negatives works, but backasswards. When I import it, it imports it in place where it is located (even though I change the path in the import window) - the thumbdrive, so I have to move the files afterwards to their location on the hard drive.