No screenshots for now, I'll get back to the camp this evening. Anyway, the glitches started with the 3 guys working with knives: their sprites went on and off when they performed the attack animation. Then a few walls started to desappeare.
Anyway, it was all caused by the glitches: the door was indeed blocked, by an invisible guard.
I tried to change map and get back. At the first try the game failed to load the map. At the second try it failed to load the save. I shut the game and restarted it. At this point everything went fine, I managed to talk to the guard who opened the door.