I also discovered a strange error when trying to use various extensions in Photos app. Many photos couldn't be opened because they were the "wrong file type" or "couldn't be found." Googling the specific error revealed the culprit to be CleanMyMac 3! That app had been ripping out some iCloud photos & replacing them with things that worked OK in photos but not with any extensions.
So I threw out the entire 23.2 GB Photos Library & let the iCloud Photo Library repopulate the Mac's library. All OK now, photos that previously generated the error message are now editable in external apps. To prevent such behaviour in future, Photos app has been excluded from CleanMyMac's processes.