After doing some more testing, it seems I’ve solved this. It was a url rewrite issue in the site htaccess.
Hello!
I am having the exact same issue. I tried a few hours ago to uninstall the plug-in, red-did all my forms and it worked only until I created a second form. I tried to change WPCF7_LOAD_JS to false but when I press Send it takes me to a critical error page. I have tried to use older versions but I am back to the newest one as none work. I have also disabled all plugins and theme but Contact Forms 7 and it still gets stuck in the spinning wheel.
My website is http://www.lac.org.pt
If you’re getting the 404 on the feedback same as me, my problem was a rewrite rule that I’d put into htaccess to force a trailing slash on all URLs. This was forcing the trailing slash on the wp-json feedback URL as well, making it invalid.
Check that in the browser console. Sounds like a different problem if some forms are working though, so you’re probably best starting your own support thread.
Fixed. Needed to set up SMTP