[Ninja Forms] Ninja form errors out

When visitor completes any of the Ninja forms on our site, for example: https://manchesterumc.org/news/

The form never gets submitted. There are no errors in the error log, The form seems to stay on 'processing', and when I watch the network request as I submit the form, the page attempts a connection to admin-ajax.php and is responded with a 404, and then makes another request to facebook.