"Must have unmodified content" or "Data differs from Server" errors.
Per EA's Support section.....
If your Battlefield 2 or Battlefield 2 Special Forces installation has become corrupt, either through the use of unsupported modifications to the game, or a corrupted patch or installation, you may receive one of these errors when attempting to connect to an online game.
To resolve this issue, Battlefield 2 and any expansions must be uninstalled from your computer and reinstalled from scratch. This replaces corrupted game files and will allow you to connect to online games successfully
If you have Special Forces installed, you will need to uninstall that expansion first, then the regular version of Battlefield 2. These uninstalls can be done automatically by using the Uninstall Battlefield 2 or Uninstall Battlefield 2 Special Forces programs located in the respective game's Program folder in the Start menu. If the automatic uninstalls do not work, manual uninstalls can be performed by following the instructions located here for Battlefield 2 and Battlefield 2 Special Forces.
Once the games have been successfully uninstalled, reinstall them as you did before. You should now be able to access online games successfully.
