Skyrim: Aggressive NPCs

Because most of the time it's quite impossible to know exactly why Skyrim behaves the way it does, I assembled a few possible culprits and/or solutions from various sources:

• You might have the cursed Ring of Hircine: if so, you need to finish that quest first [i.e. 'Ill Met by Moonlight']
  ^source

• I be[a]t them up for a long while, leveled up, ran away to the top, waited for 24 hours, slept for 24 hours, entered unarmed, and they weren’t hostile
  ^source

• On my third attempt to get Dawnguard working, Durak asked me to go the fort. When I got there, Durak and everyone attacked me and the quest was broken because the quest-giver would not stop attacking me long enough to give me the quest. On my machine, I was able to fix this by deactivating Dawnguard and making a clean save and starting the quest over again.
  ^source

• I had this problem ...half way through my dawnguard quests they became hostile to me, so I just ran before Lydia killed any of them ... I figured the problem was I had killed some guards earlier, and even though it said my bounty was removed, it glitched the Dawnguard...so my fix is go to Riften, punch a guard, pay the 40 gold, and all is forgiven
  ^source

• I fixed it by saving outside the entrance to the Vale, exiting the game, disabling Dawnguard mod, load up, immediately make a fresh save, exit, re-enable Dawnguard mod, then load once more.
  ^source

The solution with the most potential seems to be the following one, however, using the console:

• I was having the same problem, Isran and Tolan attacking me as soon as I entered the fort proper. To anyone else having this problem, add Tolan to your faction:

  "Addtofaction 0005A1A4 1 adds an NPC to the player ally faction, which will make a normally hostile NPC no longer hostile towards the player."
  ^source

  Targeted Tolan in the console, used that command, and I am now good to go.
  ^source