Forms not showing success message

I’ve deactivate caching on this site, and even before when a caching plugin was active, made sure to not cache any pages with a form. Each time someone tries to submit a form, it goes through but the person only seeing a loading icon. The success message doesn’t appear and depending on the page, they’re not redirected. This leads to them not thinking it goes through and they reload the page and submit again.

The page I need help with: [log in to see the link]