I played the Dawnguard line on one character and had no issues, then I started the questline on a different character and when I approach the gate keeper with Serana he does not recongize her and says the same lines over again ("You're not welcome here", ect..). I have tried leaving and waiting a few days in game time and going back with no luck. The only thing I can think of was done differently was I brought that character to the castle before starting the questline. Either way it is a bug that I cannot get around especially since I cannot drop Serana from following me, trade with her, or continue the questline at all. :/