Spinning icon get stuck after submit

Hi,

the plugin work almost well. The user receive the email with the download link, the admin receive the confirmation email, but when I submit the form the Spinning icon get stuck and didn’t show the confirmation message nor the file download link.

Here you can test:

http://kb.wordpresschef.it/scarica-le-book/

I’ve WordPress 4.0 version.

https://wordpress.org/plugins/email-before-download/