Form redirects instead of do nothing when form submits

I am using a Sendy subscribe form within Popup pro. The Sendy form uses Ajax. When the user submits the form, it seems to have some conflict and goes to a blank screen with one a "1", which seems to mean the user subscribed successfully.

But that is not the correct behavior. Instead, after submit the user should stay on the Popup and see a message delivered via js.

Here are my settings:
Load PopUp using: Page Footer
Form submit: Refresh PopUp or do nothing (use for Ajax forms)

Any idea why it is not "do nothing"? I enabled WPMUDEV support on the site and saved the non-working PopUp as "Sendy Test" and it is inactive. This website is very high traffic, so please do not make it active -- I think you can grab the <form> code and place it on a testing server and see the same results.