In LEGO Harry Potter Years 1-4, why are all the gold cauldrons closed after finishing game?
Solution 1:
According to teh Internetz, this bug is caused by the game failing to recognize that you unlocked the Polyjuice Potion during your initial playthrough. From what I've been able to pull together, this occurs when you play the Polyjuice unlock classroom exercise in multiplayer.
I'm afraid there is no solution for the problem. While you can go to the video room and watch the "Polyjuice Potion Intro" video to unlock the potion, you are then stuck in the Potions classroom, which just substitutes one game breaking bug for another.