The Vices of The Mother Cardinal quest Bug

Hi, I've a problem in Petrus questline: I'm trying to enter the door in the cellar, but it's always "blocked": I've on the servants clothes, but nothing happens. I've tried a lot of times, but nothing. I've also loaded a previous game (about 3-4 hours of play earlier) becouse i was locked with petrus and my last "not-quicksave" was a lot earlier.... but nothing, When I started the quest again and coming to the Cardinal basement, the door again didn't have any quest mark on it, it was only "blocked".
is this a known bug? Any tips anyway?
Thans in advance

F

Go upstairs and get the key! Don't worry I also struggled with this quest for longer than I'd like to admit. 😄