How to fix unwanted marks without darkening photos on iMac?

mirco160 wrote: … when I use the Photo program (=adjust>retouch), trying to eliminate a halo or an unwanted mark, the photo becomes completely dark, as if it were completely underexposed.

This doesn't happen for me. Do you use iCloud Photos? Do you use "Optimize Storage" with iCloud Photos?

Have you tried a re-boot of your Mac? Also try:

• 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 

If that doesn't help, 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. 

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…

Very cool! Restart and Safe restart are always a good thing to try first.