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You should upgrade or use an alternative browser. I use photos on my Mac (m4 mini) and on my iPhone. The phone syncs to iCloud, the online version shows all latest syncs. However opening the desktop version of the app it is often showing the last sync as days ago. Quit and re-open does not force a sync and there appears to be no menu option to force it to do so.

Any suggestions to get the sync function to correctly work on Photos?

My iCloud account works fine for all other apps on the Mac, it is just photos that seems not to correctly work