I’m having a similar issue since updating WP to 4.8.
The form hangs when clicking ‘submit’ and I get a mixed content warning (I am using SSL throughout my site) saying:
Blocked loading mixed active content “https://my-site.com/wp-json/contact-form-7/v1/contact-forms/108/feedback/”
Can’t find any troublesome files or links and am struggling troubleshoot this.
