I suggest sending in a ticket since this looks like a site bug and not something you two are doing wrong!
This is a bit specific, but I got my invite code from Plushieemokid and now, I am unable to invite them to my world. I get the message "You cannot contact this user". Also, when THEY try to request membership, they receive the message "You cannot contact this world". My world is set to public and we don't have each other on ban lists... Any help??