So I've completed with Dawnguard questline, I sided with the Dawnguard. Currently, I'm trying to return to Fort Dawnguard to turn in Gunmar's "Hunting the Monster" quest and every single member of the Dawnguard attacks me on sight. I've reloaded to a previous save, just before entering Dayspring Canyon, but nothing has changed. I don't have a bounty, I'm not a vampire and I'm not accidentily hitting any NPCs inside Dayspring Canyon.

I have no idea what could be causing this, but the only thing I can think of that have changed since the last time I was in Fort Dawnguard (when I picked up "Hunting the Monster") is that I now also have the Companion quest "Escaped Criminal" and I'm now married...

Any suggestions or insight would be very welcome.

You might be carrying a certain disease.

I think you just have a plain bug sadly.

If you're on a computer you could put tcai into your console to turn off their combat AI, but if not I don't think there really is any help for you at all besides evading the DG and waiting for a patch.

Check your diseases, it is possible that you had randomly become a vampire for no reason. It's a bug apparently.

People who drink a cure all diseases after every battle still become vampires and they can't figure out why. So you may be one without even knowing it?

I got attacked by Whiterun guards for no reason. I reloaded and it was fine.

do a mission fo something not dawnguard related, come back, worked for me.

One of many Dawnguard bugs sadly. I once got attacked in Castle Volkihar after turning into a Vampire Lord.