Lightroom Classic 9.1 CC (Mac) - Sync stuck in "pending" status
Lightroom classic 9.1 on a Mac.
Sync is stuck with 2653 photos "pending" (not uploading or downloading).
I have 260k photos in LR Classic, 140k synced to LR CC, hundreds of collections, and probably 200 changes pending from both ends that haven't synced.
I have rebuilt sync data several times; it always gets stuck in the same place.
The Sync Errors collection had thousands of items; after rebuilding the sync data, it hasn't re-appeared.
I have tried logging out and back in again; no change.
The status panel in Preferences/Lightroom Sync shows three pending:

Clicking on "Asset Path" takes to a web page which spins forever.
It's impossible to read the "Sync Details/Errors" since it won't expand (you can shrink te Asset Path field, but as you touch the Sync Details it reverts to the small size shown, with no useful information. I've even shortened the name of my Mac, but it hasn't updated in the log.