You may need to play with the ssl options in the Roundcube config:
// IMAP socket context options
// See http://php.net/manual/en/context.ssl.php
// The example below enables server certificate validation
//$config['imap_conn_options'] = array(
// 'ssl' => array(
// 'verify_peer' => true,
// 'verify_depth' => 3,
// 'cafile' => '/etc/openssl/certs/ca.crt',
// ),
// );
// Note: These can be also specified as an array of options indexed by hostname
$config['imap_conn_options'] = null;