The desktop app has decided to delete and download everything again. I hope this is the last time.
There’s definitely something still wrong with it. The desktop app is just syncing everything over and over again. It keeps saying it’s failing, but the files end up being there. I’ve tried all the various repairs and rescans, but it doesn’t make any difference.
This sounds like client-side sync state went stale after the files:scan reshuffled fileids in oc_filecache. The server thinks the files are fine, the desktop client’s journal disagrees, so it loops.
Two things would help narrow it down:
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On the server, while the desktop is mid-loop, grab a snippet of the log:
snap run nextcloud.occ log:tail 200 journalctl -u snap.nextcloud.* --since "10 min ago" | tail -200And from the support page send the full log archive. I’m looking for the actual HTTP code on the failing requests (412 / 423 / 507 all mean different things).
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On the desktop, what OS and client version, and what does the activity log say next to the red “failed” entries?
If you just want it to stop and you don’t mind a one-off full re-compare, the cheapest reset is on the desktop only — pause sync, remove the sync connection in the client (Settings → account → remove folder sync, not “remove account”, and don’t touch the local files), then re-add it pointing at the same local folder. The client rebuilds its journal from scratch against the current server state and the loop dies.
Too late 
I also reached the conclusion it must be the client app. About an hour ago I uninstalled the desktop app, deleted the Nextcloud folders under AppData, got rid of the old Nextcloud file folder, reinstalled the desktop app, and set it to sync. Seems to be working so far, but I probably won’t know for sure until tomorrow morning. Your way looks like it would have been a lot quicker.