It's right after the initial area....where the dude opens the door for you. Seen several times where that event is never triggered and you just have to exit the dungeon.
It can help to clear all the trash in the initial sewer up to that point. Sometimes npc's get stuck fighting them so they can't advance the quest. It won't solve all the issues, but it can help.
Had a run today where we cleared the entire first room, then cleared the entire second room and the door still didn't open... Made sure to kill everything as we went too /:
My bad...might be Weyrest 1 sorry, but yes..it's the spot where he says "We've got to get these people out of here" (I think)...sometimes he just never shows up and you're hosed.