Ok, I got this error multiple times and I decided to fix it by myself and it works now.

This is what I did.

 

IF you use the Updater (InstallIW4M):

1. Go to Local folder

C:\Users\username\AppData\Local

username = Your Windows username. obv.

2. Open a folder called IW4M

3. Delete caches.xml and double click LaunchIW4M.exe

 

IF you use the Standalone Updater:

1. Go to your MW2 Installation path

2. Delete caches.xml and double click LaunchIW4M.exe

 

DO NOT run the game using iw4m.exe since it will not update the game.

 

This will re-update the game and re-download any missing files (obv.) but it fixed the problem to me. Hope this works for you guys!

The point of this being?

The point is that yesterday a Tech Support member spread a rumor that it was a known bug, when it actually seems to be caused by outdated game files.

It's good to know what actually causes the problem.