Yes we are using plugin MC4WP: Mailchimp for WordPress. This happen when the premium plugin is installed along with the free one and ajax form submission is selected. But the strange thing is, it only throws the error exception but still able to create the entry on mailchimp list. Below are the plugin we are using and corresponding version:
MailChimp for WordPress – Premium (Version 3.3.11)
MC4WP: Mailchimp for WordPress (Version 4.7.4)
