The Brothers & Sisters of the Free Spirit had their roots in the apocalyptic theology of people like Joachim of Fiore, combined with the ideal of poverty practiced by the Beguines and Beghards. It was a group that moved from a kind of asceticism intended to break self-will, to the belief that this would lead to God in the self, to the belief that the self is God. Therefore, the liberated soul would not have to rely on any earthly religion because those restrictions were intended for "the gross". This means that there is no relationship between life in God and an ethical code.
The reason that Tauler (and other Friends like Jan van Ruysbroeck) spoke out against them was in self-defense. The Catholic church could not distinguish between the Brothers and the Friends, or any other group that called itself mystical. It looked like mysticism meant libertinism.
So, who were the Friends? Here's an article on them, from a Catholic perspective.