It does look like there is a custom signup form experience on your website. If the signup form is not working, we can definitely help you if it uses the API. To help though, we would need you (or your website admin/developer) to share the following information. Feel free to email that to us at firstname.lastname@example.org as some of it may be account sensitive.
Looking forward to hearing back from you so we can help get to the bottom of your problems.