My website, mailchimp widget in right sidebar: http://www.lauraharrisbooks.com/
I downloaded the official mailchimp plugin and put the widget in the sidebar for my theme. When I put in my email I get the confirmation sent ok, but there was no thank you/other response showing up on the page itself, it just blanked out the fields. I can see in Chrome developer tools that it's returning the correct response, it's just not showing up. It does the same if I enter an incorrect email although then it doesn't blank anything out (I assume by design).
I've tested this on Mac (Chrome & Safari) and my Android phone and it acts the same for all of them. Any help would be appreciated.