it could be that there is a server level problem causing the login not to work
to start with, lets make sure your server is configured to properly store session values.
use the session test page from the following thread:
if the session test fails, it is most likely a bad session.save_path in the php.ini file, you will need to contact the host to have them correct the problem.
if you continue to have a problem, please send a copy of the registration page, the login page and a protected page as well as the webassist/security_assist/helpergrouprules.php file in a zip archive so i can examine the code.