Hi there,
i also run into kinda same Problem as the Topic describes. But i found out what whent wrong on my installation.
Launchbox does require exactly the same cores you can find in the Emulator Core list.
For example, for SNES, Launchbox requires the: snes9x_libretro.dll core. Now if you run Retroarch it will install a newer version of the core: snes9x2010_libretro.dll.
So you have to rename that core from snes9x2010_libretro.dll into snes9x_libretro.dll.
That worked for me, i hope i could help someone out.