Facing the same problem. “Your message was sent” did not appear when send button clicked anymore but i do receive the message in my email.
Chrome console states:
POST https://www.mydomain.com/ 404 (Not Found)
send @ jquery.js?ver=1.12.4:4
ajax @ jquery.js?ver=1.12.4:4
AjaxForm.perform Request @ ajaxform.js?ver=1.0.0:61
(anonymous function) @ ajaxform.js?ver=1.0.0:52
dispatch @ jquery.js?ver=1.12.4:3
r.handle @ jquery.js?ver=1.12.4:3