Great!
Thank you very much, Danny van Kooten!
This code fixed that bug:
add_filter( 'http_request_args', function( $args, $url ) {
// only act on requests to api.mailchimp.com
if( strpos( $url, 'api.mailchimp.com' ) === false ) {
return $args;
}
$args['sslverify'] = false;
return $args;
}, 10, 2 );
