Looks like you don't have write permissions on /var/php_sessions and PHP can not write sessions on the disk (so you can not log in, because to login an user or admin you need to save it's data in a session).
This is a PHP's problem, not Osclass.
If you are in a shared hosting/server, ask your hosting provider for help.
If you are on a VPS/dedicated server, just chmod the folder /var/php_sessions andor change the PHP's configuration.
You could try adding this to your config.php file
but not all servers accept that configuration on runtime.