Submission error 'Oops'. Works when submitted twice

We have a form at Feed Me Ideas just under the image on the left hand side. If you put in your Email address the first time, hit submit you get:

‘Oops. Something went wrong. Please try again later.’

If you click ‘Sign-up’ again with the same information in place it succeeds:

‘Thanks! Please check your e-mail inbox for confirmation!’

The double opt in email is sent out without a problem.

We’ve tried:

Renewing the API key.
Renewing lists.

Nothing has changed on the mailchimp side or otherwise. Any ideas on how to resolve the issue?

https://wordpress.org/plugins/mailchimp-for-wp/