I cannot find any explanation anywhere about what is supposed to happen when the door of the Wyrm Turns is closed while a large monster sits on its base (covering at least 2 squares on each side of the door).

Some heroes smiled and said that my dragon would die. I said it would be split in two and each half would have 1/2 life, and roll a half RRB die for attack, and a 2 squares fire breath. Anyway, moving the dragon to make space had repercussions on the outcome (besides, the dragon might have not been able to expand), so I had to cordially revert a few actions and avoid the situation.

Has this situation occured to your playgroup?

I can't find an official reference anywhere either, though it has been discussed a few times in passing. The general take on the issue seems be, you can't close the door because the monster is in the way. I personally agree with this interpretation. No need to rewind or change anything, and it gives the OL a strategic tool in some quests.

If you need a more official response you can always submit it through the form.

I also know this to be the official answer (you can't close a door while a figure is blocking the line it closes on), but I too am unable to find the reference right now.

I also know this to be the official answer (you can't close a door while a figure is blocking the line it closes on), but I too am unable to find the reference right now.

It's in line with most other questions of this nature:

What happens if you "Pit Trap" a hero while he's in a space containing another hero figure? You can't play "Pit Trap" because he wouldn't be able to stop.

What happens if my shadow dragon decides to attack a hero while he is adjacent to him during movement, but there isn't room for his figure? He can't stop there to perform the attack.

Just about any time another figure is "in the way" of a potential action, that action is not allowed, with the exception of placing things, which is allowed to be done in the nearest available empty space.