Thanks a lot, Evgeni.
Line 66 of the ajax_submit.js file seems to be the issue:
var msgContainer = $(htmlPage).find('.msg-container#msg-' + id ).html();
I reached out to the hosting company and they pointed to this article:
Several forms on our site stopped working properly. When the user "submits" he/she sees the spinning wheel instead of the confirmation message.
We receive the submission on our side, but the user assumes that it didn't go through and continues to …