Okay, the most popular (but not the only) bug pertaining to the Thieves Guild in Riften is that, once people are given the quest "Taking Care of Business", Brynjolf remains at the square providing distraction to the crowds and doesn't go to the Ragged Flaggon. I'm experiencing a similar bug, which equally prevents me from progressing with the Thieves Guild quests. I've completed the "A Chance Meeting" quest and was asked to meet Brynjolf at the Ragged Flagon, but the quest "Taking Care of Business" DOESN'T appear at my quest log, neither at the completed nor the standby section. Brynjolf, on the other hand, remains at the square. I don't know whether it's the same as the popular bug, but I don't think he keeps providing distraction to the crowds, as he doesn't say the stuff he did during the "A Chance Meeting" quest, neither is the crowd gathered around him. He just stands there, walking around the square saying: "Hmm?" whenever I talk to him and he doesn't show up at the Ragged Flagon. Fixing the Thieves Guild-related bugs might not result in fixing this one. Bethesda will have to make it somehow so that the "Taking Care of Business" quest appears at the standby section of the questlog if the "A Chance Meeting" quest is completed. Has anyone else experienced this?

I've had the same problem but I have an active quest, It just leaves a marker above Brynjolf's head and he stands in his stall.

I have got the same problem as Kandosii. Brynjolf is staying at his stall and making distraction and he doesn't want to go to Ragged Flagon. I still have quest and marker is on him. It is so annoying, because I wanted to do all guilds on one playthrough and now I can't. I hope they will fix it soon.

What you and Kandosii are experiencing is a commonly occurring bug. It has been mentioned and I think it's also mentioned in that thread concetrating all the bugs, so no worries, Bethesda most probably knows about it. I mentioned my case, because it's similar to the above, but not the same. Also, because fixing the actual Thieves Guild quests might not fix this one, because, technically, I'm stuck somewhere BETWEEN the first and the second quest without having none in my quest log.