A corrupt item in the inventory can also cause a crash on act transition. Unique and set items are renamed at that time, and some items are removed from the inventory (like the BF keys of the previous act). Drop any unused keys and quest items, since none will be required in the next act. Any obvious corrupt items should be dropped on the ground. The bone ring/spear occasionally found in the BF should not cause a crash, but that is the type of situation to look for.
In Phil's case, his hero had a small bone crystal, which was locked(?), in the armour section of the inventory. Dropping this on the ground, the name changed to a hunter's knife, though the graphic remained the same. Without this item, the act transition proceeds normally.
