Join mailing list submit button fails

I thought I’d configured everything correctly but when I test the mailing list popup I’ve created and click on the Subscribe button, it just shows an endlessly spinning gearwheel.

I’ve configured integration with my MailChimp account and I’ve visited my MailChimp pages to check I have the correct field values:

First name: FNAME
Surname: LNAME
Email address: email (I tried to change this to EMAIL but Hustle didn’t let me do this).

I’d greatly appreciate help in identifying what’s going wrong and correcting it.

The page I need help with: [log in to see the link]