I've tried multiple other Cores, but it's still refusing to launch retroarch.
Also, any system that's working, shows "Launch With' and the associated Core. Even if there's only one emulator or Core.
As per my last post, I've setup a new installation of LaunchBox, using the same settings, but 'Launch With' is available. See picture in my last post. So the problem isn't how I'm setting it up, but possibly a corrupt file somewhere.
An ideas?