I planted the ring and completed that mission as Byrnjolf instructed. He asked me to meet at the Ragged Flagon.

I've found the Flagon in the ratway and he was there, too. When i approached him, he said we will talk in The Ragged Flagon but we were already in the Ragged Flagon. Then he went to his stand in the market district, never returned to the Flagon and I'm stuck. Can't even join the Thieves Guild. Also, other merchants in the market district gathered around him so i can't buy or sell anything from them too. Any suggestions or console commands to fix it?

Try this in console: setstage tg01 10
It worked for me. Had the same problem.