No space left on device

I got this on top of the page right after autenticating:

Warning: session_write_close(): write failed: No space left on device (28) in /data/project/magnustools/public_html/php/oauth.php on line 133 Warning: session_write_close(): Failed to write session data (files). Please verify that the current setting of session.save_path is correct (/var/lib/php5) in /data/project/magnustools/public_html/php/oauth.php on line 133