the first error is the cause of the others.
Warning: session_start(): open(/var/php_sessions/sess_aa2d51612e833dc563d5f27f7f48f8b6, O_RDWR) failed: No such file or directory
this means that the path for the session.save_start setting in the php.ini file is not correct.
you will need to contact the host to have them fix this problem for you, it is a server configuration issue.