There is a problem on one of my sites https://dgwwebdesign.com that uses DMXzone Mailer for form submissions.
On form submission, it redirects to the server action file instead of resetting the form fields:
dgwwebdesign.com/dmxConnect/api/enquiryForm.php

In Firefox this page lists 3 tabs at the top:

JSON Raw Data Headers

Headers tab contains Response Headers and Request Headers info.

Cheers
Dave

You need a standard HTML button of type submit, in order to submit the form properly.
Currently you are using some custom button, doing some custom action, and not directly submitting the form. That's the problem.

Thanks Teodor

I was trying to install reCAPTCHA v3 that requires custom code on the Submit button, according to the Google documentation.
I realize that App Connect only supports reCAPTCHA v2, so I have reverted to this version.
Now the form is submitting correctly with no more errors.

Are you planning on an App Connect reCAPTCHA v3 extension?

Cheers
Dave