Stuck in the quest "the mother of spiders"

So yeah, I just killed Arakaali and after that Sin and Yeena spawned in the arena but the quest status stays in the "Kill Arakaali" neither Sin or Yeena will talk to me. I tried to reset the instance but that didn't help me. At the moment it seems that I cannot progress in the game.

Okay, So I tried again a couple of times and I got it working by not using Cold Snap and it's DOT. Maybe there's something with the DOT that messes with Arakaali?

Same thing here, with my eventual windripper build. killed twice, but then Sin and Yeena just stood around and looked at me like "what have you done?"

Thinking now it's my bow, which inflicts decay.