Joomla! Documentation™

In Global Configuration->System->Session Settings, if you have Session Handler set to None you will be unable to log into the back end of your site with version 1.5.16.

There are two ways to correct this. You only need to do one of these. If you require that the Session Handler is set to None, use option (2).

1. Edit the configuration.php file and make sure the line starting with var $session_handler (at or near line 68) is as follows:

   var $session_handler = 'database';

   This will change your Session Handler to use the Database option.
2. Edit the file {joomla_root}/libraries/joomla/application/application.php as follows. On line 533 is a line with this code:

   $session->fork();

   Just below that, insert a new line with this code:

   $this->_createSession($session->getId());

   After you have made this change, lines 532-537 should read as follows:

       // we fork the session to prevent session fixation issues
       $session->fork();
       $this->_createSession($session->getId());

       // Import the user plugin group
       JPluginHelper::importPlugin('user');