Global Inboxes not syncing

Items I put in the Global Inboxes are not syncing to my other Macs or iPad.

Items I put in the Database inboxes or anywhere in a Database do sync to other Mac and iPad.

This is true for items added to the Global Inbox on any of the devices.

It happens for items added to the Finder Inbox or put directly into the DT Global Inbox.

After adding an item to the Global Inbox I see a little notification message about uploading, but the item doesn’t appear in the Global inbox of the other devices.

Manually forcing a sync in the Prefs produces a spinning cog but the items don’t sync.

The Sync settings all look correct for the Global inbox on Macs and iPad.

This has just started today, previously OK.

Using DT3.5.3 and 10.15.6 with recent supplementary update.

Any suggestions?

I cleaned the Inbox Database (by right click on Inbox in the Prefs) on one of the Macs and now the Global inbox is syncing again.

Why would this happen? How can I prevent this kind of disconcerting anomaly ?

You haven’t specified what kind of sync you’re referring to.

Dropbox sync store.

Stopped working again this morning.

CANCEL above for now, Started working again, doing more investigation.