I can't start the Thieves' Guild quest "Under New Management".
I was able to complete "Darkness Returns" without any problems. Karliah did not randomly disappear or vanish or anything. I didn't immediately go back to the Thieves' Guild. I did a couple of other quests and then went to the Thieves Guild. I talked with Brynolf fine, but it didn't give me the prompt for the quest. Now all he says is "Sorry lad, I can't speak....I've got important things to do right now" (or whatever it is he says, something to that effect). This is my second game doing this, and in my first game, I had no trouble starting this quest. I looked online for tips on this, and it said as a last resort that I can actually attack a member of the Thieves' Guild and/or kill said member and then do the "Reparations" quest. Upon finishing that, it'd be possible to start the quest. I did that (I made two saves so that I wouldn't lose the prior game), and it didn't work.
I don't know if this has anything to do with this problem, but I actually haven't advanced very much in the main quest story line. I started the quest "A Blade in the Dark" but haven't actually finished it. Does this have anything to do with my Thieves Guild problems?