Answer:
Make sure you play free play and play the level called "The Restricted Section"
Pick a gringott
Go to the part where Filtch is trying to grab you and strangle you.
Go near the stairs on the right and there is a thing for a gringott to put his key into and that should unlock Filtch when you open it.
Hoped that worked. If it didn't notify me and i will fix it shortly.