Whenever I try to enter the common room, I can't as I don't know the night password.
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If the ball party quest is active still, you need to talk to a girl in the middle of the floor in the upper part of the ballroom. It's also skippable by simply talking to the Gryffindors in the same area, and then doing the part with Hermione.
If the ballparty is finished, there's definitely a bug here