Hustle does not add subscribers to MailChimp list

I have set up Hustle to accept email addresses that should be added to a MailChimp list. I've added a MailChimp API key, and it correctly showed the MailChimp list within Hustle.

However, emails entered into the forms are not added as subscribers to my list, even though the Hustle form shows the success message.

Within the MailChimp dashboard, under API keys, I can see that requests have been made against the API, but the response is marked as "n/a" (see screenshot).

I have tried creating another key and re-connecting the form, to no avail.

This is somewhat urgent, as I will have a speaking gig next week, and the site was prepared to accept lots of subscribers at that point.

Thanks for looking into this!