Doom3 will not let you connect to a pure server if the checksum of the pack files (.pk4) the server uses does not match the checksum of your pack files (.pk4). You will receive this error if you have used a tweak that required you to unpack .pk4 files or your pack files are corrupt.

To correct simply copy the original .pk4 files from the Doom3 CDs to your Doom3 / base directory or reinstall.

You may also receive this error if the server is running a different id software patch to you, has customised the original .pk4 files or has packs that you must have in order to play on the server.