99.9 percent sure it's the ioboard. Found this from the tech docs
Force
The game displays “MAXX loading” then “invalid key”.
To test whether the I/O board is being detected by the motherboard, turn the game off and put DIP switch 6
(DS6) to the ON position. Turn the game back on. If “Maxx loading” is followed by “invalid key” again,
this would indicate that the motherboard is not detecting the I/O board.