After upgrading to Asterisk/FreePbx 12 happenz the next error
PHP Warning: session_start(): open(/var/lib/php/session/ failed.... Permission denied (13) in /var/www/html/admin/config.php on line 68
Praticaly when you login to freepbx web interface, and select any item in the menu, you will return to login page !
To solve
chown -R asterisk:asterisk /var/lib/php/session