Moved site from sub domain to primary and now get error

session_start() [<a href=‘function.session-start’>function.session-start</a>]: open(/tmp/sess_c7015b9685c83fb4f7839e9fd36ca463, O_RDWR) failed: Permission denied (13)

I have my /tmp set to 777

Check if you have open_basedir restriction in effect (e.g. with phpinfo()). I once had a hard time with it. It required me to explicitely define a session.save_path in php.ini.