I fast traveled to Fort Dawnguard, and I'm a member of the Dawnguard. Suddenly, when I step inside, everyone is immediately hostile towards me and attacks me on sight! What the hell is going on?!

I'm not infected with Sanguinare Vampiris, I'm not wearing Vampire Armor, I don't have any diseases...

Has this happened to anyone else? Is it a known glitch?

You just got punked!! But seriously, I havent heard that one before. Strange.

Most likely you accidentally killed someone who was Dawnguard and now they hate you. Hopefully thats not the case, but if its true, you're screwed...

Well I found an earlier save where it doesn't happen, and I haven't lost any progress, so I'm deleting those saves where it happens.

But the only thing that happened was going from Fort Dawnguard to Riften, for the quest where you warn Riften's Jarl. Vampires attacked inside Riften and I killed them. That's all. I didn't do anything else except roam around Riften before going back to Fort Dawnguard.

Unless...using the face sculptor was the other thing that I did. To get into the Ragged Flagon I had to kill the hostile NPCs in the Ratway. But I doubt that that would affect anything, unless Dawnguard considers those as Murders.

Pretty lucky man that would have svcked.

Did you do this before you had to meet Esbern? If the problem happens again stall your Dawnguard quests and try and finish up the main quest at least until you get Esbern. This hasn't happened to me since I already finished the main quest but it might work.

This character isn't doing the MQ. I purposely avoided Jarl Balgruuf so that dragons wouldn't spawn. And I didn't see or kill Esbern while I was down there, that much I'm certain of. I killed the first two guys, the Lowlife, and the pugilist guy, that's it.

I can't help but think it has something to do with Murder. The Radiant Quest "Prophet" warns against "murdering" the target. Even though these guys weren't the target, maybe Dawnguard is programmed to "know" when we murder in the Rift Hold???

Is it the quest where you kill the vampire that infiltrates Riften? Because I killed him right in front of everyone and got a bounty, paid it, and the DG still like me?

Yes, it's that quest called A Jarl's Justice. So paying the bounty for the Murder of the "target" doesn't bother the Dawnguard? Why would they tell me not to murder him then? Unless its the game programmed to save you the trouble of guard conflict...