In order to fix the issue navigate to Start > Control Panel > System and Security > Windows Firewall > Allow a program through Windows Firewall and check the box next to Ragnarok Offline 3 2nd Rev.
After that, from the Advanced Settings select Inbound Rules > New Rule. Check the Port box, click Next, select All local Ports and follow the self-explained steps. Also, make sure that your security program doesn't block the connection. If the error persists, reinstall the game.
Note: If you can't find the game in the "Allowed programs and features" list, press the Allow another program button, navigate to the installation directory, select the executable file and press Add.