CKFinder 2.0 failed to call a hook error

I had installed ckfinder 2.0. and set CheckAuthentication function to return true. Samples were working fine.

Than I've put session_start() at the beggining of config.php. Until than ckfinder doesn't work anymore. When i started it I get a message CKFinder failed to call a hook: and button OK.

Does anybody know what is wrong? Is this a bug in ckfinder 2.0?

<?php
/*
 * ### CKFinder : Configuration File - Basic Instructions
 */
 
session_start();

/**
 * This function must check the user session to be sure that he/she is
 * authorized to upload and access files in the File Browser.
 *
 * @return boolean
 */
function CheckAuthentication()
{
   return true;
}