This is similar to the one that I've seen a lot on the internet but mine seems to be unfixable.

After I complete the task where I place the ring in the guys pocket, I go up to Brynjolf and speak to him, he gives me the money for completing it and then I get notified that the quest is complete. But then when I go down to the ragged flaggon all he does is say "hmm" I've tried everything I can to fix it. Including going to an old save file, but the same thing happens again.

If this isn't fixed there's no point in me playing anymore.