empty session path

Hi, I've looked a while here, googled but found no hint.

My site https://www.MYDOMAIN uses sessions without problem, the session path is empty, however.
For testing i use version.MYDOMAIN and sessions are lost. here the session path is empty as well.

sess_cache folder has '777'

My config file is

$config['sess_driver'] = 'files';
$config['sess_cookie_name'] = 'ci_session';
$config['sess_expiration'] = 7200;
$config['sess_save_path'] = FCPATH . 'sess_cache';


$config['sess_match_ip'] = FALSE;
$config['sess_time_to_update'] = 300;
$config['sess_regenerate_destroy'] = FALSE;

$config['cookie_prefix'] = '';
$config['cookie_domain'] = '';
$config['cookie_path'] = '/';
$config['cookie_secure'] = FALSE;
$config['cookie_httponly'] = FALSE;

What might be wrong with my setup?