alex242: When I press F12, a file appears in the appropriate folder with the png.tmp file extensions. If I get rid of the tmp the file still doesn't work. I've looked around fora solution, but didn't find anything that would work. Can anybody help me with this/point me towards a solution?
I guess the game from where you want to take screenshots was made with the Unreal Engine 4/5?
If so, I can confirm that there is a bug in the Galaxy overlay.
The solution for now is to use an older overlay where that bug doesn't exist. I'm using the overlay from version 2.0.59 which is working fine. You can download it
here.
Press Win+R and copy/paste the following...
%PROGRAMDATA%\GOG.com\Galaxy\redists
Replace the "overlay" folder with the one from the zip archive. Restart Galaxy and try if it works now.