I haven't seen this bug reported on this forum, but I have seen it on the nexus. Ive got the Steam version of Skyrim and Dawnguard, lots of mods (can post list if needed), and once I enter castle dawnguard on the first quest, I get aggroed by Tolan, which makes Isran and the other guy attack me too, for no apparent reason. I do not have any bounty, I am a werewolf (which some people suggested might have something to do with it, but not always), and well, that's basically it. It's really annoying as I can't really do anything to get over this quest, and will probably keep getting problems. Any thoughts??

It's gotta be the fact that your a werewolf. Vampires and Werewolves, are not a big friend to the Dawnguard.

No, I know for a fact that it isn't coded like that, as many people are doing the quests as a werewolf with no problem =/. I have found however that Vigilants of Stendarr attack me wherever I go, and seeing as Toslan is a Vigilant... could this be related? Any solutions?

no as the only way to make all vigilants attack you on site is to attack one and my lvl 60 has killed scores of them and I never had this problem.

Okay, no idea as to what the cause of the problem is, however I did solve it by using the "setrelationshiprank" console command on the guy, he didn't attack and neither did the others, worked like a real charm actually.