[Forminator Pro] Forminator Pro not working - https issue?

I created a simple form using Forminator Pro which has a couple of hidden fields and a numeric input field. When I try submitting the form I get the following error:
An error occurred processing the form. Please try again

Looking at the browser console it appears to be trying to load an ajax request over http and being blocked, as the site is https.

Mixed Content: The page was loaded over HTTPS, but requested an insecure XMLHttpRequest endpoint '.../wp-admin/admin-ajax.php'. This request has been blocked; the content must be served over HTTPS.