I have the same problem on PS3. I tried to join the Stormcloaks, killed the Mist Wraith. I don't remember whether I came back or if I went on in the main quest. Now, in Season Unending, I'm trying to arrange the truce conference. Angeir tells me to talk with Tullis and Ulric. I talked with Tullius and he agreed, but Jarl Ulfric tells me: "Speak with Galmar, you'll be going on a little errand with him." He won't talk about the quest. Galmar says: "I don't know how you convinced Ulfri to talk to the empire, but . . ." and won't take me on the errand.
