Was doing a dung, and found an unpassable room. For some reason, the empty square (pointed to by the arrow) couldn't be walked on, meaning there was no way to get through, and get the key / open the door required for completion. There were no alternate routes, and directly clicking the NPCs just gave the "You can't reach that." error.
Not sure if this was a one off or an actual broken room, but let's hope it doesn't happen again.