Here is the messed up world and the error log
Thanks, that helps a lot in figuring out what it was.
It's not the bug that was preventing load of older worlds, this one is a bug in the world file itself: the world.dat file is completely blank. There's no way to recover than that unless you have a backup (which I'm hoping you do).
I've never seen this, and really don't know how it was managed. The bug that started this thread wouldn't have done it because it happened during load; the world was never fully loaded such that it could be saved back in a corrupt state (and even there it wouldn't have written nothing).
If you're able to find a way in which this can be made to happen to a world, then that would be a bug in the game and I could fix that to prevent it happening again. As this case stands, I don't know if it was a bug in the game or something weird happening in the filesystem (some kind of permissions denial) or what.