I have a pretty big problem, which has pretty much destroyed the dawnguard expansion for me.

-Minor spoilers-

I have told the mothpriest to go to the downguard fort.
When I then enter the fort myself, the priest becomes hostile and starts attacking everyone including me, I can't talk to him while battling and when the downguard soldiers have defeatet him he sits on the ground grasping for air, and the game says he is busy, I can't talk to him or pickpocket him.

The longest I have gotten is to get him to read the elderscroll. I achieved this be exiting and entering the fort again and again, but after that, there's nothing I can do. in other words, i'm stuck.

Has anyone else expirienced this and maybe have a solution to this bug?

I remember in the beta forums there was a bug like this. I don't know if it's the same one and it never happened to me so I don't have a fix unfortunately

This happened to me in the Beta.

You need to go back to the part where you have to go into the cave and release him from his vampire captors. Its not confirmed yet, but many were talking about having Serena accompanying you will cause him to go hostile. Your best bet is to have her wait outside the cave...or even in Dragon Bridge.

The first time around, I did not find him in the the magic gate, but rather across the river under the entrance. Your best bet is go just go in and blow through the cave and tear it up. You need to loot the orb from the master vampire and use it at the terminal to unlock the Moth Priest. If he's not within the purple magic walls, just reload the save.

He needs to be in there when you release the wall spells. He will have a thrall placed about him causing him to go hostile. You must be defeat & release the thrall here in order to progress correctly.

I believe the issue is some individuals were encountering him without the thrall in the cave, thus when he travels the Fort Dawnguard, the scrpit kicks in for the thrall, making him go hostile.

That is exactly what happened to me, the priest wasn't in the magic cage, he was running around, killing vampires.
I will try this at once.
Thank you very much, it seems like this is gonna work.

best thing to do is just go back a few saves...

Get the illusion spell called pacify and dual cast it at him this will calm him long enough for you to talk to him and progress through the story line it's only a temporary fix but you can do this as many times as you want to keep him non hostile.

Pacify or Harmony if you have got them. I had exactly this problem, and those were the only solutions I could come up with. I would suggest though only casting harmony in the first main chamber, otherwise you knock nick-nacks and sundry cr*p all over the place.