Sorry, I wasn't clear enough. Behind the link in my post it is explained that the nVidia+AMD combo, aka in my case original GT 120 and Sapphire RX 580 together makes Apple Photo app act erratically. After removing the GT 120 I could repair the previews inside Photos app by editing them (and then canceled the editing). It all seems to be working alright now without GT 120. Apple Photos pictures and previews do work again as expected.
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