This error occurs when GameGuard file authentication fails due to an altered or missing .INI file. You will need to repair your game client through the NC Launcher before attempting to log in again.
This can be done by clicking on the Scan File option at the bottom of the NCSOFT Game Launcher window.
Note: If you have the Launcher set to start the game automatically, you will have to run the launcher once, close the game or Alt+Tab out to the Launcher mini window and adjust the settings from the Settings button in the top right-hand corner of the windows. Once done you can restart the launcher and you should be able to run the File Repair.
If you continue to experience an issue, please contact Technical Support by logging in and submitting a ticket HERE.