I may have found a solution which, unfortunately, involves patching the plugin’s code.
You need to find all instances of:
$api = new MCAPI_13( $apiKey );
and change it to
$api = new MCAPI_13( $apiKey, true );
Adding “true” seems to establish a secure connection (maybe MailChimp doesn’t accept unsecure connections anymore?)
These are the instances I found:
autochimp-settings.php: lines 95 and 294
autochinp.php: lines 559 and 924
Hope this helps