I kept getting the error message below. All folders have the right permission. What is causing the this problem?

Warning: session_start() [function.session-start]: open(/var/www/vhosts/, O_RDWR) failed: Permission denied (13) in /var/www/vhosts/ on line 13

Warning: session_start() [function.session-start]: Cannot send session cache limiter - headers already sent (output started at /var/www/vhosts/ in /var/www/vhosts/ on line 13

try this:
* login to your ftp with an ftp client
* find /var/www/vhosts/
* chmod it and check all write or chmod 777 for it.
this should solve it
let me know if it is ok.