After clearing the farportal boss, it may happen that, later in the game, the farportal room is closed again.
The quest is supposed to be done (and so the butler does not open the door with the horror boss), but the room is closed.
I finally found a way to fix it, by editing the appropriate quest engine and setting the quest as not done, but is is rater tedious.
And you have to refight the boss horror
