The World File Mismatch error usually means your Rust client has the wrong version of the map or a bad cached map file.

To fix this, you will need to clear your Rust map cache.

1. Close Rust completely.
2. Open your Rust install folder through Steam.
3. Go to the maps folder.
4. Delete the files inside the maps folder.
5. Restart Rust and try connecting to the server again.

You can usually find the folder here:

Steam\steamapps\common\Rust\maps

After clearing the folder, Rust will download a fresh copy of the map the next time you connect.

If the error continues after clearing your map folder, try verifying your Rust game files through Steam, then attempt to connect again.