Ok. I am replaying the game, and I'm at the point I need to slay Alduin for the first time (just after reading the Elder Scroll). No matter how much I use Dragon Rend on him, I can't harm him. My summon don't hit, my arrows don't hit, mele attackis register no gamage too. Am I missing something or am I just victim of a glitch? Reloading the autosave does'nt seem to help, so I tought maybe I am forgetting something, but as far as I can remeber, brigning him down with dragon rend was sufficient to make him vulnerable.

Thatr is being fixed in the 1.9 patch. It should be here maybe next week.