Demon invasion.

Do you have any saves from before this point? Did loading a save and changing the hero's turn order help?

Added after 56 seconds

It clearly wasn't intended... Do you have a save where I can take a look?

I forgot about the NFH mod. I have it installed, but I also have a mod that adds a skill wheel to the game itself (but the first time, there was no skill wheel, and I still couldn't enter without casting a spell).
I have a save, although I started replaying this mission a second time, hoping that this would make the bug disappear, but it didn't.