I had this same problem. There was a fix for me. My solution was my configuration url was wrong.
Click Configuration > Site URL: Just put your domain name eg https://YourDomainName (or http if you’re http) eg https://domain.com or http://domain.com
I had mine as https://domain.com/mautic (I installed it in a sub folder). Ignore the subfolder, just put your base domain name.
Correct solution was found here
https://www.mautic.org/community/index.php/7552-v-2-8-0-404-on-mautic-form-js/p1#p22721 by MxyzptlkFishStix,
which was found https://www.mautic.org/community/index.php/7100-mautic-form-js-failed-to-load-resource/0
Good luck to you.