Ok the mistery is solved. For being given the quest by the guild master there have to be bandits in town. Just save at nighttime and try acces the market: if the transaction screen appear wait a little bit while in town and then try again, while saving before every attempt. When you try access the market from menu and suddendly find yourself in town freely, thats it. Load that last save and wait till daytime, and now the guild market can give you the quest.