This error indicates that EAC was unable to authenticate your connection.
You should ensure that you always launch Rust by using the Steam games library as launching Rust using other methods will result in EAC not launching which will lead to errors.
Here’s a list of things you can try to resolve this issue:
1) Close Rust and wait for up to 10 minutes
This will allow EAC to refresh your cache which may resolve your problems.
2) Verify your game files:
3) Repair EAC:
- Open your Rust folder (right click on Rust in Steam library > Manage... > Browse local files) and go to EasyAntiCheat folder
- When you're in the EAC folder, click on explorer's navigation bar and type EasyAntiCheat_EOS_Setup.exe repair and press the Enter key
- If you've typed it correctly, new modal window will show up asking to grant administrator rights for the EasyAntiCheat process. Allow it
4) Restart your PC and internet connection:
Turn your PC as well as your router / modem off and then on again.
5) Verify connectivity to Easy Anti-Cheat:
See EAC’s guide here
6) Check for windows updates
Update your antivirus / see if temporarily disabling it has any effect.
8) Delete ‘RustClient.exe.eac’:
Go to your Rust directory at *\Steam\steamapps\common\Rust\EasyAntiCheat and delete ‘RustClient.exe.eac’ if it exists
9) Check for ‘forbidden applications’:
Check towards the bottom of your "output_log.txt" in \Steam\steamapps\common\Rust for any applications that EAC has marked as "forbidden" and try stopping them from Task Manager. Find more information on the output_log.txt here
10) Perform a fresh reinstall:
If none of the above steps have solved your issue then you should perform a fresh reinstall as shown here