I upgraded my plug-in for my sign-up form and started getting this message on my WordPress blog in place of the actual sign-up form:
There was a problem loading your MailChimp details. Please re-run the setup process under Settings->MailChimp Setup.
I re-entered the API key, etc., trying to get this to work, to no avail. Had to remove the associated widget from the sidebar to get rid of the error message.