I just installed this plugin today and I am having the exact same issue. My API key works, I can select the list, etc. But when someone fills out the form, they get “failed to add user – 400′. The subscribers page states “failed to add to mailchimp [400]”
When I check the API calls on MailChimps site, I see:
Result – X
The requested resource could not be found.
Result – X
Your merge fields were invalid.
