Invalid MailChimp List ID: 3b27444534
It looks like this issue is known by the maintaining developers for the mailchimp extension. Please see https://github.com/veda-consulting/uk.co.vedaconsulting.mailchimp/issues/129 and run through the steps they've suggested on there. 

If it leads to a fix, it would be great if you could come back and post your solution on SE!