First - I already know there is a similar thread.

The first time I played with Dawnguard installed I had no problems. When I approached Castle Volkihar, with Serana in tow, the guard at the gate recognized Serana and opened the gate. The quest objective Lead Serana to her home (completed) and Speak to Harkon were in my journal.

I started a new game. This time when I approached Castle Volkihar, with Serana in tow, the guard at the gate DID NOT recognized Serana AND DID NOT open the gate. The quest objective Lead Serana to her home was in my journal uncompleted.

Using cheat Setstage to complete Lead Serana to her home did not resolve the problem

To continue, the only solution was to use the TCL cheat (no-clip) to walk through the closed gate. The remainder of the Dawnguard quest went OK.

The likely cause of this BUG is I went to Castle Volkihar BEFORE I started any of the Dawnguard quests. If this was the cause, then this IS a BUG and needs fixing.

My suggestion for a fix is to remove the gate from Castle Volkihar when the quest objective Lead Serana to her home is assigned.

but then the gate keeper would have no job T.T

but yeah this bug even happens on consoles. Reloading doesn't typically help either.