Was just checking around if there was something new added to the level 4 starting area and decided not to go yet to the area where the Beast is so I checked out if the level 5 entrance was still at the same place. Yes it was but now I'm in an endless loop of black screen and sounds of fire and low-pitched thumps(?) as heard in the video (don't mind the picture, in reality it was just a black screen, OBS left it there from a previous recording. And no, there was no UI either) while not being able to do anything except Alt+Tabbing from the game. Before the video's events my character screamed a few times as if falling to death, once I had entered. I also had my companion following me (who's supposed to be dead according to my previous thread) and he was low on health.
Soon I'll try to restart the game and see what's my last checkpoint and what happens if I try to go to level 5 again. Maybe even try a new save but that will take some time.
EDIT: Restarted the game. Last checkpoint saved was in level 5 after transition. Tried moving between level 5 and 4 and it seems to be working normally again. What happened to me could be just a very rare case.