Here is an example of a session error
[Sun Dec 26 18:01:35 2010] [warn] mod_fcgid: stderr: PHP Warning: Unknown: open(/var/lib/php/session/sess_kv4u1j6ehbrml1uq0tf7el7326, O_RDWR) failed: Permission denied (13) in Unknown on line 0
[Sun Dec 26 18:01:35 2010] [warn] mod_fcgid: stderr: PHP Warning: Unknown: Failed to write session data (files). Please verify that the current setting of session.save_path is correct (/var/lib/php/session) in Unknown on line 0
I found this error, in the "error_log", that is stored in "/var/www/vhosts/xxxx.com/statistics/logs"
This write inability make that sessions don't work in PHP