If WordPress is asking for credentials, use the following:
Hostname: <leanservers_host> (e.g.: acacia.leanservers.com)
Username: <leanservers_username>
Password: <leanservers_password>
Protocol: FTP
Alternatively you could add these lines on your wp-config.php file (this is recommended and fixes the issue once and for all):
// ** FTP SETTINGS FOR AUTO-UPDATE ** //
define('FTP_HOST', '<leanservers_host>');
define('FTP_USER', '<leanservers_username>');
define('FTP_PASS', '<leanservers_password>');
