RawTherapee crashes when trying to edit NEF files

I had my post written but I deleted it as I did post my findings briefly in the bug report you mentioned. There I mentioned why gdm played a role and was working under it (not for a reason I thought).

I don't like C++ and I don't do much in it. I really don't like debugging it. But I was intrigued enough to push it through here.

It seems locales are even bigger pain there. As you can find a reply in the bug report there is PR 257255 with Andrew giving a workaround that worked for him.
My reply and investigation in 6876 gives a reason why that segfault is happening.

In the end that function is not properly written because it expects it will always find a profile by a key that is strongly locale sensitive. That's not ok. But at least there's a solution. Running LC_ALL=C.UTF-8 rawthereapee should work for you too.
Ha, that worked!!
I just set "env LC_ALL=C.UTF-8 rawtherapee %f" in my menu for Xfce and it works!
The "%f" portion was already there, I didn't want to remove it.