By the way, I've found the cause of this issue. If you have a summon out when you complete the black circle encounter near the tree, when the Advocate and friends appear they instantly become hostile and attack. If the summon dies and you flee the battle they become non-hostile again, but it's too late because the Advocate will not talk to you and the quest breaks.
So do not have a summon alive when you complete the battle. I tested this twice, it's definitely the cause.
