Happened first with 12.3, rolled back to 12.1, ditto, after update to 12.4 it's still existing. Never had it in my setup before, and like I mentioned, I didn't change anything in that setup for almost 2 years now, everything is exactly the same.
Since it happened the first time, it's doing it all the time. I added Atari ST games, went to manage emulators, setup the emulator and associated platforms, and it gives me this error. Tried then with ZX Spectrum, exactly the same. Tried also with different emulators, no change.
In the beginning I thought it was because RetroArch had the cores for those 2 systems set as emulators, but after removing them from RetroArch, the problem still exists. I even physically removed the 2 cores (Fuse, Hatari) from the RetroArch cores folder.
The emulator.xml file also doesn't show any of those 2 entries.