Had the most unusual experience ever when approaching the Dawnguard Castle to start MQ for this expansion. Character has never had any encounters with vampires yet. Agmaer immediately was hostile, so I had to defend my character thought it was just a rare bug or something, further approaching the main castle and the rest get hostile. I'm on PC (as indicated in my signature) only have USKPs for all my content & When Vampires Attack mod installed.

Had anyone ever encountred this issue before? or is this just one of those rare bugs.

Cheers.

Happened to me, too. I believe it's because you're too "low level" to start the quest.

I was around lvl 7 without Durak asking me to join.

Then Agmaer greeted me with "his pa's axe".

No, I had a character that "discovered" the castle when he or she was level 1 and nobody was hostile out there. I bet the uskp are the culprit, if it's the only mods the op uses...

I am using all the USKPs and have been through the DG questline two times with no problems. For what it's worth, my character was above level 10 both times she initially approached the castle - although I'd be surprised if being lower than the prereq level would cause hostility. I was not running When Vamps Attack for either of my playthroughs. Sorry no real help here except to share what may be relevant data from my game.

Okay, try this: on the loading screen do not select an existing save, open the console and use

coc DLC1DawnguardHQ01

It will create a dummy player directly inside the castle. See what happens both inside and outside the castle; if people are still hostile, the problem is not in your current save but in your game, mod related probably. If the player doesn't face any hostility, the problem is with your current character. He or she did something that raised that hostility.

Do you have a bounty in The Rift? The guys at the Dawnguard become hostile when you do.