i'm having a problem with this quest as well, except mine is serana is locked outside of the gate and after i read the first scroll, i have to go talk to her. but she's locked out and i'm locked in.
i told her to wait at fort dawnguard so i could go find the location on my map for ancestor glade (everytime i tried to go there with her, she fell off a mountain or got stuck). so i locate it, go in to see what its about, and lo and behold serana appears out of no where. i initiate dialogue, but the only 2 options i have are "we should part ways" and "trade items". no wait, or follow order, and she appears to be stuck in wait mode cause she wont follow me.
so yeah back to the bug. i have to go speak to her, per the quest requirements. but the gate locking me in wont let me out to get to her.
so i guess i can either try "pushing" her all the way through the cave.... or go to my last save which was 3 hours of playtime ago, after just getting the quest to go find her mom