Cannot edit some photos

Please bear with me for a moment while I explain the situation...

I created a new Photos library on a new iMac 27" in Jan. 2018 and imported about 3000 holiday photos. Due to prolonged illness I did not edit them for quite a while. Now going back to the old photos, I can see them in full glory, but cannot edit them - any operation from the edit menu results in no change at all. I can move the sliders back and forth as much as I like, but the image doesn't change.

Trying to export the "unmodified original" results in "The file P1010xxx.jpg couldn't be opened because there is no such file." Duplicating the photo within the Photos app results in a blank thumbnail picture.

Using the Find Any File app I can see the files in the "masters" package and open them with Preview.

I have imported many newer photos into the same library since early 2019 and edited them successfully, no problems there.

I tried rebuilding the Photos Library several times - no change.

Apparently something happened between early 2018 and 2019, but what might it be?

How safe are my many photos for long term access when I store them ion the Photos Library and Photos can't access or export them after a while? When I extract them from the Masters folders, do I get the original/unedited version or the edited one? What should I do to protect my photos and my editing efforts for the long term?

Mac Os 10.14.6 / Photos 4.0