Just a reminder that providing specifics on, sharing links to, or naming websites where ROMs can be accessed is against the rules. If your post has any of this information it will be removed.
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If you have a save file, I suggest you delete it manually (or move it elsewhere) and try again. (Save Files are stored in "C:\Users\<User>\Saved Games".) I'm guessing this is a problem with how the game stored your previous save file, as this is an error from Ruby itself and not anything in Essentials (from what I can find).