Backend Acess Problem

When you have a login problem, this might be a symptom related to Joomla 1.5.16 or higher.
They have tried to increase the security and introduce a bug that is reported in the bug tracker.

If you have a joomla 1.5.18, perhaps you should have roolback the sources /librarires/joomla/application/application.php or put the session line in comment at line 530.

joomla bug tracker
/*

$session = &JFactory::getSession();

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