Migrating from Lightroom 6 on Mac to CC on Windows: "The catalog seems to be an exported catalog"

I am trying to migrate my Lightroom catalog from Lightroom 6 on a Mac running OSX 9.5 to Lightroom CC on a PC running Windows 10 Pro. I get the error message "The catalog seems to be an exported catalog or has been copied". The help page wants me to fix file paths in Lightroom 6, which I have done. Both the (indeed copied) catalog file and the photos are on an external hard drive.

What's wrong and can this be fixed, or do I just have to import all the photos?