Mailchimp Form - No confirmation with Popup Pro

Hi there,

I have been using the regular (non-pro) version of the WP PopUp plugin with integrating an Mailchimp signup form. Everything worked fine incl. a confirmation message within the popup after clicking the submission button.
Yesterday, I signed up to the Pro version of the popup plugin in order to show different popups to different visitors.
However, since then the confirmation message as part of the plugin is missing. The popup disappears and visitors see the landing page.

What I did: I just copied the shortcode into the main popup content form. The popup loads via wordpress ajax as I have the same signup form already placed in the page footer - and error messages that should be shown in the plugin appear in the footer when using "loading via page footer".
By using Google Analytics real-time data, they show that apparently the "Thank you for subscription" page is loaded.

Thanks for your kind support!

In case you want to take a look at the situation please feel free to visit