Hi on1ev,
I quickly browsed through the schematics, it seems the keyboard consist of board A1 with all the keys, and board A2 with a 8741 (or 8751?) scanning the keys and interfacing to the main CPU.
Now the explanation of the error (invalid key) says "You pressed an unde fined softkey", so maybe you just have a stuck key? You could probably test this by disconnecting A1 and A2.
cheers,
marc.
