Photos on macOS not syncing with iCloud

Everything that has a MOV file or has an associated MOV file (i.e., a Live photo) opens in Photos on my Mac with this icon in the lower right corner:

Double-clicking the icon displays a message like this (for videos, it's similar):

The underlying reason is that the Photos app on the Mac doesn't sync with iCloud.

Every other sync with iCloud works fine (e.g., Contacts, Notes, Reminders, etc.). Photos sync fine with iCloud on the iPhone (logged in with the same Apple ID). Mac and iPhone are on the same Wifi. The Photos app on the Mac is the only application that stubbornly refuses to sync.

I spent 4 hours in chat, phone calls, and VNC sessions with Apple support personnel this weekend, so any wrong settings etc. can be ruled out. There's ample storage left both on the Mac and on iCloud. The usual steps have all been taken, like restarting the Mac, cutting its power for > 20 seconds, turning iCloud sync on and off for Photos, etc.

It seems the Photos application "remembers" it couldn't sync at some point, and refuses to recognize that it can do that again now. Maybe there's caching involved, I don't know.

Per Apple Support's suggestion, I changed the iCloud sync settings, like this:

But that doesn't change the situation, of course, since Photos will still have to sync to retrieve the originals from iCloud. We're not talking about a lot of files, or even large videos, so the sync should be quick:

Photos doesn't sync any new photos or videos, but for the existing ones, only MOV files or photos with attached MOV files are affected.

Any ideas what's going on here are greatly appreciated.