Move your Mods folder to the Desktop and delete localthumbcache.package. Then test the game.
If it works now check for updates to your Mods and CC. Add it back a little at a time while testing until you have found all that are broken.
Still doing it without mods and thumbcache and across saves.
New game no mods, still does it. I placed a (base game built) home from build mode, exited, created a family of 5 (4 randomized sims and a cat) to put in it to sort of mirror my initial conditions, loaded into live mode, attempted to open build, couldn't, went back to build from the world menu, and once more encountered the immovable living room thing.
I just tried repairing my game, but still, same thing.
