iCloud for Windows will not download photos or videos - Error 0x8007017C: The cloud operation is invalid.

I have always had problems with the new MS Store version of iCloud for Windows, but as of today it will no longer sync any photos or videos from cloud to PC. I can click on a jpeg, mov file, the download will start, and then I either get "Error 0x8007017C: The cloud operation is invalid." or it just gets stuck at 98%.

I've also tried switching the on-demand setting for each file to "Always keep on this device" but the selected files never download.

Inside MS store, I "Repaired", "Reset", uninstalled and reinstalled." I also rebooted my PC, verified that the files are indeed present on the icloud server. I have killed and restarted the icloud task. I'm completely blocked from using the app at all now. I've got 6,000 photos and videos to download, and the icloud browser approach is incredibly tedious and buggy for this kind of volume.

This app has a rating of 1.9 on the MS Store. Many other users report the same problem. It's Apple's integration with the Windows on-demand files API that is buggy. Dropbox and other apps that use this Windows API work fine - it's an Apple coding issue, not a Windows issue.

Anyone have any idea how to resolve this issue? I am speculating that perhaps there's a file mapping issue between server and client, as just before encountering this error I had deleted several thousand files client side. Perhaps the diff database is now corrupted? Is there some way to force iCloud to resync the local folder? I would have thought the reset would have done that.