Thanks very much for the reply Nightwalker! Please let me make sure I'm seeing the same thing that you do:
[spoiler:When I go through the portal, I enter a room with a table with a book which starts "The portal seems to be unstable...", an unopenable gear lock labeled "Godportal", and one door which is unopenable. When I started the mission, I was in a totally different room with an obviously fake door, which may be the one you mention, attached to two other rooms (one is a kitchen, and the other has a ladder to the attic where the portal is). If I am supposed to be teleported to the original room, this is not happening. Instead, I'm teleporting to the "new" room from which there seems no escape.
Does this sound even remotely familiar, or am I the lucky recipient of a very strange but extra-special 'mission feature'?