I'm having the problem where the moth priest is attacking the dawnguard as well. It seems like the glitch occurs when you first encounter the moth priest. Your mission tag reads: "talk to the moth priest".
I've been through the questline with both the vampires and dawnguard now, and it seems like it only happens for the dawnguard side. I believe when I went through and decided to join the vampires the moth priest was actually IMPRISONED in the magic barrier. For the dawnguard, he's just walking around near it. Your supposed to have to kill the vampires there before freeing him, and for some reason if you can't do it this way, he'll attack both Serena and the dawnguard agents when you encounter him again later.
I've tried everything to glitch it "back to normal": downing him and all the dawnguard agents, dismissing Serena, downing the moth priest and yielding multiple times, etc. The only way I can get it to stop is casting the master level calm spell over and over again in dawnguard castle while I'm there. Lower level illusion spells DO NOT WORK, as his level seems to be fixed somewhere between 30 and 40.
At least I was powerful enough with illusion to make it through the dawnguard side of the quest, but for anyone who did not invest in illusion perks, this glitch would make it impossible to finish the questline. This glitch appears to be very widespread, and it's quest-breaking. Bethesda needs to fix this NOW!!