I went out and killed all the keepers after talking to Valerica and she told me to follow her. But then she just stands there and I can't complete the quest. Any ideas on what I should do?

Do you still have Serana with you, or has she managed to get herself lost? Valerica will do nothing without Serana being there. If you lost her you will have to go find her.

Nope. Serana is right beside me. Restarting the quest multiple times didn't help either.

If the barrier is down just walk into the Boneyard and see if the quest progresses.

I did, but Valerica and Serana didn't follow me. I battled Durnehvirr and the game told to me to talk to Valerica. After I spoke to her she commented on the dragon's defeat and told me to follow her to the elder scroll. Then she just stood there...again.

The scroll is in the Boneyard, in an alcove opposite I think to where the entrance door is. Maybe you could go check if the scroll is available? No idea why Valerica is stuck, are you perhaps playing on the PC? DLC does not seem to run well with some mods.

You could always save your game and reload, or leave the Soul Cairn and come back. Or once you have saved just try the Unrelenting Force shout on her and see if she gets the hint you are displeased with her, just try and blow her in the right direction.

Thank you! You reminded me that I was playing on the PC. The problem was that I had too many mods loaded. Once I unchecked them from the loader, everything worked!