Form not working with https

Since I have changed from http to https my form isn’t working. When I click on the sent button the page reloader symbol appears permanently but more doesn’t happen. No error message, no page validation message. E-Mails aren’t sent. In the browser DevTools I see no error messages, but the ajax response of the form contains html code. Without https I get as ajax response a json object with mailSent status true/false. The rest of the site is working well.
What do I have to do so that the form work with https too?

Thanks.