I just entered Act 2. As soon as I click on the first enemy I encounter, the game crashes to desktop.
i was having similar problems. while searching the msg boards for help, i came across several threads saying that some items (such as battlefield keys) are removed from your inventory when you move between acts.
so i loaded up a saved game and checked my inventory before leaving act 1 AND upon entering act 2. sure enough, my bf keys and some other items were removed. the funny thing i noticed this time was that my bow had become unequipped between acts. after equipping my bow, i successfully attacked a mob without any crashes.
tbh, i'm not quite sure of the exact cause of the crash. it probably was a combination of many factors including weapon type/monster type/friend or foe (the first monster you see doesn't actually attack you...it only becomes hostile(thus crashing) when you attack it).
anyways, i'm off to act 2...hope this helps others with similar problem.