as i wrote, it's not only with sdlmame. it crashes with single mac games, in my case.
have you tried to connect x-arcade after sdlmame is launched, not before? it works for me (didn't test it in sdl), despite it's not a comfortable solution.
anyway, i opened a support case with x-gaming and they say
"Because the X-Arcade works just like a normal keyboard when connected to the computer, it doesn't have any firmware upgrades."
i also asked about the usb adaptor, like geoman. waiting for the reply.