Lightroom CC: After "succesfully migrating", 4300 photos missing from Cloud and local storage out of 5000

I have migrated my Lightroom Classic catalogue to Lightroom CC, following all, I mean all the steps described on your website (https://helpx.adobe.com/lightroom-cc/using/migrate-to-lightroom-cc.html), in the correct order, after having updated both apps to the latest version, and after having upgraded my plan to include 1TB of cloud storage. (PC is a Surface Pro 4 with Windows 10 Pro, with all latest updates; I had previously imported 76 photos in Lightroom CC, to kick the tyres; those photos were not previously imported in Lightroom Classic, and they were still present in Lightroom CC when I started the migration). The Lightroom Classic originals and the catalogue and everything else is stored on my C: drive. I have changed the storage preferences of Lightroom CC to store a copy of all smart previews locally on the C: drive (not in the same folder as Lightroom Classic). I have then changed the storage preferences of Lightroom CC to store the originals on an external drive (M:), and to keep a copy of all originals there. You know, just in case. The Lightroom Classic catalogue I decided to migrate has 5000 photos in it, for a total of 79,5 GB.

Now: Lightroom CC told me that migration was mostly successful, with 7 items not migrated. There were 4993 items in Lightroom CC, and albums were migrated correctly. I thought I would wait for the Cloud sync to finish and then check what was missing, and check that everything was OK. Old school, counting bytes, files, and then checking folders one by one.

I left the laptop running. When I came back, the sync was "completed". What I found out is the following:

⦁ Lightroom CC: only 797 photos in total; the originals folder on the external drive (M:) is only 15,2 GB.

⦁ Cloud: All Photos shows a count of 5000 items. Space used in the Cloud is 77 GB; a very big majority of photos (I guess 5000 - 797?) are not visible. There is a placeholder with a cloud icon and a light bolt symbol on them. When clicking on one of those, I go on the view for that image. The same cloud icon and light bolt symbol are there, and beneath the following is displayed: "Syncing file '' from unknown. Added on 2/13/18." . On the photo info panel on the right, it actually displays most information correctly: filename, camera, shooting settings, dates, copyright, etc. But the image itself? Nowhere to be seen.

⦁ Lightroom Classic: of course I fired up the Classic version, to see whether my files were actually lost! The images are still there (and can find them back on C: as well), but now it has synced with the cloud, and has created folders in the library view for all days in which the 5000 photos were shot (lots of folders). They are, as expected, all empty. If I go in one of those folders on C:, in them there is always a file named info.lua, and the content of that file is always

info = {
 name = "unknown",
}

⦁ Lightroom Mobile on iOS (latest version on an iPhone 8 Plus): same is in Lightroom CC

Obviously this is not expected behaviour. And: I can't import twice the same catalogue. And: my original catalogue is now a mess. As I write I'm looking at the website for Luminar 2018.......