Upon completion of all the bits and pieces of level 3 in Chapter 1 of IWD (Vale of Shadows) the party pulls a lever that opens a couple of big doors controlled by gears on the NW wall. Pressing the 'C' key to show clickable areas displays a giant purple trapezoid, but it does not register clicks to move to the next level.
See attached savegame. I removed the custom portraits from the attached game, let me know if that causes any trouble. All characters should lack big or small portraits.