I encountered a bug where you can't trigger the fight with Saella. Her character sprite is motionless and can't be interacted with, and there doesn't seem to be a trigger for the cutscene. It doesn't seem to matter how you get into the brothel, by fighting or using the ticket. I also tried talking to the NPCs and that didn't seem to fix it.
Edit: I double checked a few things and noticed that the quest to get the book is duplicated three times, once with "Infiltrate the Thieves Guild" and two times with "Infiltrate the Pink Brothel". Maybe that has something to do with it?