Random Photos become blurry

When I edit photos in Photos, multiple thumbnails turn blurry after editing. When I select the picture and open it, the photo in full size is blurry as well. When I click 'edit' the photo becomes sharp again. After saving it the photo and thumbnail will stay sharp but the photo next to it will become blurry.

Also, drag and drop to Finder will freeze Photos completely.

The only way to get out of this crazy situation is to use 'Revert to original' for the affected photo's.

I did the 'Photos library repair tool' procedure to fix a corrupt database but that doesn't do anything.

Please help, I am completely stuck because of Photos.