I loaded a different save that's progressed farther and I didn't run into any failures to load. I did try to get to that level before doing the sewers root quest on the save that broke.
I hope it's only a problem with 6.7. I made another character and saved in the sewer, and now I get the same "Didn't spawn a dungeon room" message but I spawn in a strange void. I cannot play that save anymore. There are a few invisible walls, what looks like the root sky box only visible in certain places, and an enemy level tag slightly above the floor I'm on. It seems to be happening on levels with a root on them. I am also abusing followers to power level to all of the abilities I want, but I don't see how that would cause this.