<a href="http://kaiporter.com/tramadol-hydrochloride-capsules/">tramadol hydrochloride capsules</a>This is what I found at the top of each AJAX response of Contact Form 7. Of course it is corrupting the JSON data. It's clear that this is the direct cause of the issue you're seeing.
The same link is also found in the HTML source of front-end pages of your site. I suppose this link is added for the purpose of link building (obviously black hat one).
I don't know who has added the link, but in the contact page, it was inserted into a div element that the theme handles and it was only content of the div, so I thought it's most likely that the theme added it. I may be wrong about this, but you can make it clear by switching theme temporarily.