Similar problems with my second playthrough. I went with the mages during my first playthrough and was pretty much bug free beyond some clipping issues.
Playing on nightmare mode. I tried a wide array of graphical settings and nothing really seemed to remedy the issue other than persistence.
By loading an old save repeatedly until the characters rendered right, I finally got them to work. Kind of.
It seems that there are a lot of graphical glytches involving the area of the Envy demon fight, even when the characters aren't stuck in that da Vinci's Marcus Vitruvius pose.. A couple of times the character I was controling would fall through the landscape, sometimes the screen would go black but I could hear the fight continue.
Eventually, after multiple saves, I got through the fight. However, that was just the beggining of the problem.
Whenever I crossed the threshold to talk to the templars after the fight, the screen would go black and the loading image showed up. A few times the conversation between the templars would initiate, but the screen remained black and despite the dialogue wheel showing up, I was unable to manipulate it and move on fromt he scene.
Eventually, after having to fight Envy several times on nightmare (challenging fight when it was a roll of the dice on how many of my characters would be playable), I got the black loading screen, but rather than enter the fortress after loading and talking to the templars, I reapeared back in the center of the Envy fight area. From here I was able to again cross the threshold and talk to the templars.
Really weird bug. I think it must have something to do with that threshold (instance or whatever) between the fortress and that courtyard area where you fight envy.
Edit: GTX 675MX ; I7 3840-QM 2.8ghz; 32gb DDR3