I have done this quest 4 times and never had an issue with it, but it bugs out on people. But I have done it with scads of other players and when the place was empty.

Two things to try, one is easy, one is tedious.

1. If there is an item to activate, look in your inventory and manually use it when the game asks you to press a button to activate something. In this case, the Urns or containers your supposed to drink from. (Not sure if this will help in this case but something to remember for other quests that might glitch)

2. Abandon the quest and restart it. This fixes most issues with quests.