To fix this, you can edit the php.ini file and set the value of "session.save_path" properly.
If you don't have any access to the php.ini file, you can edit the "includes/init.php" file. Search around line 10 for this code:
exactly above that line, add this code:
change "/var/tmp/session" with your own folder path. The folder need to be a valid and writable.