It basically had a "Signing in error" when I launched again for half a second, after which it signed in as usual, but proceeded to corrupt the save nonetheless.
Or, sometimes GFWL changes the encryption key for the save hashing, when it fails to sign in, so it renders all the previous saves invisible... I tried rolling the Xlive folder that keeps the encryption keys (presumably) back a week - didn't help, so I guess the save got bodged too.. Tried rolling everything back a week, all folders that have "Live" in their name.
I don't have a backup set up on my Steam folder, unfortunately, so I couldnt roll the save back. But even if I did, unless it made a reserve copy yesterday night when I finished playing, it would roll back to being empty anyway..
And as I said, I simply didn't get to manually back it up, since I was not making that much progress... Such a load of rubbish all of this is.
I guess it is worth giving it a go (not like we are getting a patch any time soon), but just make sure you back up the game every like half an hour... then you are guaranteed not to lose, well, more than half an hour at a time.