I had the same problem, but changing permissions around didn't help. It turned out to be an issue with session_save_path in php.ini: http://www.php.net/manual/en/function.session-save-path.php
This problem is php.ini problem:
>hosting control panel >>PHP Scripting
edit php.ini and go to this place:
; where MODE is the octal representation of the mode. Note that this ; does not overwrite the process's umask. ; http://php.net/session.save-path session.save_path = "/var/php_sessions"
session.save_path = " /var/php_sessions "
session.save_path = " /home/users/web/xxxx/xxxxxx/public_html/phpsessions "
you can "autoinstall" Mediawiki using install script site; https://secure.installatron.com/apps#cmd=browser&display=mediawiki
anyway you have to change your " session.save_path" address