We need to create a MailChimp user for the client before an account can be created. If there's no mailchimp account, assume that the account creation email can't go out to them so it blocks creating the new account. Probably something happened since this user was first set up some years ago to cause it not to have a not to have MailChimp user set up, such as an email change or something to that effect.
You can use the Create Mailchimp User button to set them up in MailChimp; this should fix the issue:
