When installing Mantis in a shared hosting environment, you may get an error message like this one on the Mantis login panel:

SYSTEM WARNING: session_start() [function.session-start]: open(/var/php_sessions/sess_ac5acafaecb612345675d30123456789, O_RDWR) failed: No such file or directory (2)

This message is likely caused by the fact that /var/php_sessions/ is not accessible. It can be resolved by adding to the config_inc.php file the following line
$g_session_save_path = '/home/users/web/b880/ipw.myaccount/sessions';.

Replace the path above by one accessible by your environment.