The reason for this is a typo in the Form Action field of the Genesis. (This is very common :))
1. In the MailChimp eNews Form Action field, all you need is the highlighted text shown here. (The URL after action="copy the entire url")
2. In the Genesis eNews Extended Widget, the most common cause of this is pasting the incorrect code into the Form Action field.
You should only paste the URL in the Form Action field. Do not paste the entire <form action> code. See below.
In the Form Action field, it's common to see this. (This is wrong and will cause an error when the form is submitted)
<form action="//myblog.us10.list-manage.com/subscribe/post?u=ce20862sdfc29a1adabcd0d35a&id=3a1408d1" method="post" id="mc-embedded-subscribe-form" name="mc-embedded-subscribe-form" target="_blank" novalidate>
Here is the correct entry for the Form Action field.