The chasing echoes quest, is not starting, I have completed prophet (as a vampire), but serana doesnt talk with me, I have tried using startquest DLC1VQ04 or setstage DLC1VQ04 10, serana talks with me, but the quest doesnt start (it doesnt appear in the journal), I also tried going to the location I was suposed to go, I killed the skelletons, but the door was closed, and needed a key to be opened, so I cant continue with the quest or something, and I need the elder scroll to continue with the other quests in the vampire questline...

I have also searched for this problem, but anybody else seems to have it

That's what i have found and you should try:

Did you kill Alduin [dragon ending first] before starting Dawguard???,

if you have the Elder Scroll [Dragon] in your possession before starting Dawnguard it misses up dawnguard, if this isn't the case then a mod missed up the scripts for Dawnguard.

Or, you could just go to the locked door, open console, click on the door and type "Unlock".

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Are you using followers in your journey? Because i remembered when i faced Serana outside the castle for that quest, after the dialogue nothing happened for long time.

Quest was accepted just after i sent home my follower and talked to Serana again.

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Try to use completequest to just bypass the Soul Cairn altogether.

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Tried the "reset stage" command?

I havent killed alduin yet, I havent had the dragon elder scroll till now, that I got it for the quest, and yes, I have a follower and my follower is serana, and what is the reset stage command? I have tried typing reset stage, resetstage but nope

Edit: Tried the unlock command, and it doesnt work, I guess because that door needs to open when the quest starts...

sorry i meant setstage, but you have done it already....

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Do you have the game updated to the latest unofficial patch?

or maybe you have some mod's installed ???