Photos fails to export modified version of an original photo on Mac

I think that I might try going into Edit mode, doing something simple, and closing-- it might work. Another simple thing to try would be duplicating the picture and trying with that. Duplicating might clean up the file. It's worth a shot.

 To narrow things down, try these steps:

• Restart the computer (of course)
• Re-start in Safe Mode. This bypasses certain potentially disruptive processes, and it often helps. Safe Mode is different for different computers, so see this: 

           Start up your Mac in safe mode - Apple Support 

Those are the easiest things to try first. Then try these:

• Use another library if you have one, or make a small test Library with a few pictures in it and see if the same thing happens. You can create a new Library by closing Photos and then option-clicking on the Photos icon.
• Make a new user and see if the same problem recurs with it.  This is a bit harder, because a new user can't access a different user's stuff. You would need to move your Library out of your own user's folder up to the general "Users" folder. 
• Rebuild your Photos Library--close Photos and re-launch by option-command-clicking the app icon. If you use iCloud Photos, it’s possible that this will cause a re-sync that lasts long enough to make you nervous.

The idea is to find out if the problem is with the Library, with your account, or with caches and login items, and stuff like that. One of these steps may fix the problem, but their failure to work also helps figure out what's happening.

Let us know what happens…

Did you edit the photo with the Photos library or use an external editor?

If an external editor was the edited file extension the same as as the original? If not Photos will not know that it's been edited. Also you will get this warning about Live Photos:

When you drag the photo from the library you will get the edited version but at a marginally higher JPEG Quality than a High quality export, i.e. 44% size reduction by dragging vs 46% for High export.

Try the following in order as needed:

1 - launch your library with the Option+Command keys held down to restore/repair the library. Test to see if the problem persists.

2 - launch Photos with the Option key held down and create a new test library. Import a couple of photos, edit them and test to see if the problem persists. If not close the test library, open your current library and test again to see if the problem persists.

3 - if the problem still persists log into a new, admin account on your iMac, and run Step 2. If the problem goes away then you have some 3rd party software that is causing the problem which we will address at that time.

danielfrommo wrote: …the odd thing is that, if I dragged the modified photo to the desktop (instead of exporting it as I usually do to dedicated folder on the desktop, the modified version copied just fine.

A word of caution-- you can never be sure what you get with drag and drop from Photos. As you point out, there are lots of ways (quality, size, format, etc) that a file can be exported, and there is no guarantee from Apple about what you will get-- and we've seen it change with different OSs. The easiest thing for Photos is to use the Preview copy, which is lower resolution. I use drag and drop for sending texts, for instance, because I don't care if it's lower quality.

I had hopes for rebuilding the Library-- what happened?

I forgot to ask originally if you're exporting by this method:

Just so we're all on the same page.

Thanks for the update and great sleuthing! 👍