Form not working, only when viewing as draft

Hi,
A client of mine has a weird with the Contact Form 7 plugin. The form works fine when executed from a draft. But when the article is published the form breaks.

When you click on the submit button it doesn’t seem to send any information because not even an error message is popping up.

I’ve checked the HTML, disabled all plugins and scripts but the problem keeps popping up only when viewing an article when it has been published.

The only difference between the two is the permalink.

Form when published”
<form name="" action="/page/formulier-test/#wpcf7-f615-p643-o1" method="post" class="wpcf7-form" novalidate="novalidate">

Form when being viewed as draft:
<form name="" action="/?p=643&preview=true#wpcf7-f615-p643-o1" method="post" class="wpcf7-form" novalidate="novalidate">

The published form can be viewed here:
http://www.ideo-nl.com/page/formulier-test

Tonight I’ll check the form when the site is using a different theme. That way I’ll know if it’s something in the current functions.php file. But I thought maybe somebody else has had this problem before or knows what’s up.

The website runs on a Windows server by the way..

https://wordpress.org/plugins/contact-form-7/