Hello,

So, first could you check if any captcha enable for your site? As maybe a wrong setting there (which could explain the form not being submitted.

Then, if no captcha enable, you can enable error reporting on Maximum in global configuration of Joomla, and enable the debug mode.
Try then to submit the form, and do a screenshot of any message or error reported.
You can then post this screenshot here in "private mode" so that only you and me will see it (confidential padlock icon in editor bar)
After test, don't forget to turn error reporting on default and disable debug mode.

Best regards,
Cyril

Right, this javascript error comes from Joomla.

So, it would be good to check if you don't have an override in your site template for the validate.js file.
A way to confirm that is to test you site with the default Cassiopeia site template (if no issue, so the issue is due to your site template).

If it does not work as well with Cassiopeia, so it could be another third-party extension overriding Joomla core validate.js, but with outdated code.
or it could be a broken joomla update, so it could be good as well to try a re-install of Joomla core files.

Hope this helps?

Best regards,
Cyril