Please load a save before entering the last room and try again.
Otherwise, you can also send us a savegame so we can test what went wrong. You can find it here: ...\Documents\My Games\Enderal\Saves.
We need the -.ess and .-skse files, please put them in an archive, because the forum does not support -.skse directly.
(if possible, please no auto- or quicksaves, but saved manually)