After my web server provider has switched from PHP4 to PHP5 in July users had problems to log in and new users couldn't even register. It took me a while of work to find out the actual reasons. It turned out that the SESSION variable wasn't properly stored and I had to call
in Postnuke's includes/pnsession.php (right after ini_set('session.auto_start', 1);)
in order to get it working. Thanks to TiMax
Now the login and register functions should work again. Sorry for the inconvenience and thanks for your patience.