Found the solution myself
I found the solution by clicking around.
Instead of copy/pasting the icon directly in Getinfo, I clicked 'New original', navigated to the icon and opened it in Preview. I then copied the icon and pasted it the way I had tried before, but now successfully. So far so good. Finally I had to create a new path to the program itself, but the new icon stayed in place.
Bingo! All is good now,