At this point all I can think is to try it on a fresh install of RTW. Make a copy of your existing game so you keep everything you have, then do an uninstall and manually delete everything leftover (you will still have that copy you made). Then install RTW and BI fresh.
Then I suggest you take that fresh copy of the game and make a copy of that too. Keep it aside as a backup. Then in the future you can use that to make more copies of without having to uninstall and reinstall the game. It makes life much easier.
Then install ATW to a fresh copy of the game, and see if you still get that error.
You might also try downloading ATW a second time, just in case something went wrong with your original download.