Update on my problem: Which has gotten worse. I read somewhere that actually Hylea was the one who ends the quest. Then I rushed to the temple and talked to Hylea. And she ordered me to solve the "dragon" problem again?! Well, I solved it already. Nothing changes.
I killed the Sky dragon at act III but the quest didn't get updated. So I can't end it. The dialog option just doesn't appear when I talk to the quest giver NPC. Is there anything I can do besides backtracking to my last backup save :/ Thanks.