I have the same issue with the newsletter block connecting to MailChimp, but it is erratic:
1) Some new entries post fine to MailChimp and some fail, for no obvious reason.
2) When I am notified of a failed posting (unsaved form submission) I immediately test entering a fake subscription and it posts to MailChimp fine, so why are some entries failing?
3) I have tried a MailChimp disconnect/reconnect a number of times, no joy.
4) I have opened a case for the issue.
5) Any ideas?