A new synced collection has only synced some of the images the rest have been hanging for several days

I am using Lightroom Classic 15.5.1 on an iMac. I have approximately 40,228 photographs already successfully synced over then years.

I created a new synced collection containing 803 JPEG photographs. Syncing stopped with 364 photographs remaining and stayed there indefinitely.

Lightroom Sync Activity shows the first affected photographs as “Uploading — [file path] — Image.”

View Sync Status on Web reports no sync problems, and there is no “All Sync Errors” collection in Lightroom Classic.

I tried removing individual photographs at the head of the stuck queue. This allowed syncing to progress temporarily from 364 to 362 and subsequently to 356, but it then stopped again on another photograph. A newly created synced collection also remains queued behind these photographs.

I have generated the Lightroom Classic Sync Diagnostic Report.

I have not used Rebuild Sync Data or reset the cloud library.

Anyone out there having this problem?