The above-mentioned error exists when you mix the current logged in Ragnarok gaming files to an existing RO folder which may be connected or belongs to an Unknown Ragnarok Online Server. Additionally, The client version of such server is either outdated or it’s created in order to run on an old version gaming because of system requirement privilege.
Few basic steps that may help you:
Download the gaming files from the server’s site.
De-Activate your (Anti-virus) just for the installation duration.
Install the game in a fresh/new folder.
Update the client using the patcher.
Run the game.
Follow my instruction on your own risk.