playBunny: Considering the rules don't define the MDU, nor does the game enforce it, I doubt that The player could not make a legal move with the second die refers to the MDU.

Note that the rule above mentions legal moves as well: If the player cannot make a legal move, he is notified about this situation by a message "You cannot make a move" and must pass this move to his/her opponent by performing an "empty" move - clicks on one of sending buttons. Such move is displayed as "pass" in the game notation.