Where and how are you using the Form and the form widget? Do you get error on every page where you place the form widget (just for testing)?
Are you using the same or different domains for admin app and front end app? If different, make sure you have valid SSL certificate and both apps are running on HTTPS.
Are you able to reproduce the issue using the sample Dancing Goat project? If yes, please share with us the code and steps so we can take a further look and debug the issue.
I think there could be some custom script used on the home page. If it was a global issue, like wrong presentation URL, CSP headers in web.config or custom headers, then I would expect that it will not work anywhere on the site.
I would also check the maxUrlLength setting.
Were there made any customizations to the form widget?