Hi, I had this problem this morning too with a shared server that has an invalid save_path set. In the end I found the fix as creating a folder specific to session saves (as I always have to anyways), and then the following lines at around line 157 in /includes/session.inc (the code there didn't work so easy, though it was on the path I used to /session/store/ so slight mod worked.
// Changed session.inc at line 156, version 2.2 release
//ini_set('session.save_path', dirname(__FILE__).'/../tmp/');
ini_set('session.save_path', $path_to_root.'/store/session');
session_save_path($path_to_root.'/store/session');
// The extra session_save_path is the way I usually fix this. Not sure if that was necessary after the ini_set.