I'm experiencing the same problem, it started this month (the module was working before then), last email added to a list is July 18th, 2018. No email sent, and no subscriber added to any list.
This might be related to Mailchimp's recent announcement.
"On July 30, 2018, we’ll make a security change to the MailChimp API that will disrupt calls made by your API client or integration unless the proper configurations are in place. This change comes as part of the new security standards that are required by the PCI Security Standards Council.
Beginning on July 30, HTTP calls will automatically redirect to HTTPS. Before this date, we strongly encourage you or your integration’s developer to review the information below to make sure your calls continue to work as intended.
If you manage the API client
Verify that your API library or operating system that runs your API client can either make HTTPS calls or follow redirects. As a best practice, we recommend that any necessary updates support TLS version 1.2."
I've already set my website to https before July 30th, however the error still persists. FYI I have the latest Shape 5 Mailchimp Module installed.