There are times when you will need to modify the defaultsession.save_path which is hard coded into your PHP confguration. When using Plesk, the PHP.INI file is shared however you have control over many of the flags.
First, you need to create a .htaccess file in your document root (/httpdocs), it is within this file that you add your custom PHP session save path flag, eg:
php_value session.save_path “/var/www/vhosts/yourdomainname.co.uk/httpdocs/tmp”
This flag would set the session save path to your /httpdocs/tmp folder, simply ensure this folder exists. If you are manually installing Joomla you may need to change the session.save_path. We recommend that you use the Joomla installation script found under “Web Applications” in your hosting control panel as this automatically sets the sessions.save_path.