Prosites Plugin Activation Error


I am getting the following error after I activate prosites plugin in a clean wordpress multi site install, without having any other plugins, also using only the default wordpress theme. I am also sure that I give necessaary permissions to /tmp folder (755, also I tried with even 777)

I am using Cpanel, Linux server.

Error message is as follows :

Warning: session_start(): open(/tmp/sess_4c87d71b24bd74d85a40cf1e2c06f833, O_RDWR) failed: Permission denied (13) in /home/bulunmax/public_html/wp-content/plugins/pro-sites/pro-sites-files/lib/ProSites/Helper/Session.php on line 29

Warning: session_start(): Cannot send session cache limiter - headers already sent (output started at /home/bulunmax/public_html/wp-content/plugins/pro-sites/pro-sites-files/lib/ProSites/Helper/Session.php:29) in /home/bulunmax/public_html/wp-content/plugins/pro-sites/pro-sites-files/lib/ProSites/Helper/Session.php on line 29