I already said in another thread that NPCs aren't talking to me once I had finished the game, but it's not the quests but instead it's my rank.
I tried to join a group for the missions on the starter island, since I can't make the mission start myself (the NPCs say odd things to me, see below). It refuses to start the mission once I'm in the group, and sais to the group leader that I don't have a high enough rank for the mission.