Hi,
Thank you for the details, it helps a lot. Please check this documentation for troubleshooting.
The error message 404 on the second screenshot very likely indicates either a mod_rewrite rule issue, or some sort of an access issue.
Try opening this URL when logged in as the regular user: https://masscircle.com/wp-admin/admin-ajax.php
It should print a white screen with either a 0 or 1 or -1 on it. If it shows a 404 page, then it means, that something is preventing the WordPress core ajax handler access.