Guild Wars 2

I just encountered a bug that blocks progress inside the Nightmare Chamber. I defeated the opponents in the first room, and now I stand there with a blocked exit in front of me and nothing to do. The only “mobs” sharing the room with me are some respawning white spiders, but nothing else happens. There is nothing I can do to advance the mission any further.

That sounds like you are in one of the portal chambers. There should be 3 on the blocked path. The right one will let you through. The wrong one will down you. The spiders will walk through the right one but I am not sure if they keep doing it.