General 400 error when posting

When I try and post I get this message:

GENERAL
400
at: https://eatdrinkcapetown.co.za/wp-admin/admin-ajax.php?
action=proxy_atd&_wpnonce=e8e1f22559&url=/checkDocument

I’m using the Dyad theme.
I’ve tried everything I can find on the internet regarding this.

The website is https://www.eatdrinkcapetown.co.za

Thanks for any help you can give!