When the website visitor clicks Submit on your form, it posts the information vi ajax to the server and the server responds with a little bit of JSON. That JSON is getting corrupted because your system is set to display PHP warnings and the PHP warning message is getting mixed in with the JSON response. This is a fairly common problem. You need to turn off WP_DEBUG (or otherwise make PHP warnings stop displaying to the screen .. some folks like to just make that stuff go to a file rather than to the screen, which is what I do .. if it’s set to go to screen then it messes up that ajax transaction).
This is what gets returned from the server:
<br />
<b>Notice</b>: Undefined index: ebd_downloads in <b>/var/www/dietaanticancro.it/wp-content/plugins/email-before-download/email-before-download.php</b> on line <b>432</b><br />
{"mailSent":true,"into":"#wpcf7-f2871-p2890-o1","captcha":null,"message":"Grazie per il tuo messaggio. \u00c8 stato inviato.","onSentOk":["document.getElementById('wpm_download_1').style.display = 'inline'; document.getElementById('wpm_download_1').innerHTML='The link to the file(s) has been emailed to you.'; "]}
That first part with the “Notice: Undef…” blah blah is the problem. To be clear, it’s the warning text itself that is the problem. What the warning is complaining about can be ignored — that’s not a problem. You just need to make the warning stop or at least stop going to the screen. Turn off WP_DEBUG or make it go to a file or whatever.