The MailChimp API-reliant aspects inside of your plugin are not working, so instead of signing people up to a MailChimp list, the DHVC Form plugin just returns an error message.
MailChimp is no longer supporting its 2.0 API, and has deprecated it so that only MailChimp’s 3.0 API works now. Could you please update your code to use MailChimp’s 3.0 API?
Here’s a link to Mailchimp’s changes: https://mailchi.mp/7a37f04b6eb2/api-export-10-and-api-20-retirement
Thanks a bunch!