It's your Mailchimp Signup Form Code that is causing the issue.
Regretfully, we cannot provide support for third party plugins or scripts as our support policy in the sidebar states due to the fact that there is simply no way to account for all of the potential variables at play when using another developer's plugin or script. Because of this, any questions you have regarding setup, integration, or troubleshooting any piece of functionality that is not native to X will need to be directed to the original developer.
I would recommend you use
Email Forms Extension Instead.
You can install it via
Pro > Validation
Thank you for your understanding.