Basically I did the first quest from Dawngaurd, and was asked to bring Serana home, but when you bring her to the gate, the guard won't open it. This is a workaround using the Whirlwind Sprint shout, and a wooden plate to clip through the gate. This should work on Xbox One as well (PC users can use console command).
Once past the gate you can enter and things continue as normal. I believe this bug happened because I discovered Castle Volkihar before starting Dawnguard.

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