I was having trouble finding out how to begin the quest "A Cornered Rat" after finishing "Diplomatic Immunity" , so I searched some youtube videos. In all of the videos I watched, it said "Quest Diplomatic Immunity Completed, Quest A Cornered Rat Started" after you talk to Delphine at the end of the quest. However, when i talked to Delphine at the end of "Diplomatic Immunity", it did not say that I started "A Cornered Rat".
I'm pretty sure this is a bug, because when I try to talk to Bryjolf to ask him where Esbern is, there's no option for it.
Is there any other way to start this quest? If not I guess I'll have to leave the main story alone for now and pray to god that they fix it in the next update :/


Edit:
From reading what the rest of the community has been saying, I'm pretty sure that this bug occurs when you speak with Brynjolf before completing "Diplomatic Immunity"

OR, it might be because I tried to kill everyone in the Ragged Flaggon before I ever spoke with Brynjolf. I feel like the above is more likely though.

I'm having the exact same issue. I also can't join the thieves guild. Any quest that has me talking to that guys whose names starts with a B in Riften just won't work. Anyone else having issues?

I had that issue with Brynjorn (B guy) at first too, but the 1.1 patch fixed it. Have you gotten it?
I was able to join the thieves guild; I've completed its quest line and I'm still unable to trigger "A Cornered Rat"

same problem here, i saw a few more ppl with this same issue, no solution yet tho :/

I can join the thieves guild, but the main quest is dead.

Bump.

why did you bump it 6 minutes after a post, when it was already at the top of the forum?

Good point.

That being said; bump.

I got 1.1 this morning. I went to Riften and saw Brynjorn heading into the Ratway. Finally, I thought. That fool went half way there, turned around and then came back up. What a tease. No thieves guild or main quest for me, not yet at least.