The title says it all.

I just finished the first mission of Dawnguard, went on horse to tell Isran about what happened at Dim Hollow Cript. After that, I exited the place, got on my horse and went to see the gate near Fort Dawnguard until a magical block says you cant go any further or something like that, I unmounted my horse, shot an arrow to the other side, then mounted it, but while I was mounting it, my character animation was mounting far away from the horse then appeared alright on him, traveled a little again to forth downguard, and while on road the horse suddenly kept running but in the same place whitout advancing a single bit, if I jump with the horse it stays falling for several seconds and if I move, like I said, it animates the horse running but stays in the same place whitout advancing as if some invisible barrier was on his way.

Reloading the game just helps a bit, and after some mts. advanced, the same happens. When I unmount it the ride horse option doesnt appears, and I can walk through the horse as if it was invisible but I cant still hit it with my weapon. Anybody had this problem? Some advice? I have HF and DG installed just to give more info on the matter.