Hi there, doing a quick scan on Google returned result where other users had same issues regarding the configuration of the hosting. Take a look at these links that hopefully will provide you with a solution on sorting the hosting out.
http://stackoverflow.com/questions/5104065/php-session-handling-errors
https://wordpress.org/support/topic/unknown-php-errors-bottom-of-bloghelp
http://forum.sp.parallels.com/threads/subdomain-with-fastcgi-php-sessions-not-work-chmod-777-var-lib-php-session-not-help.112600/
Best regards
Chris