first this is to check the serves session management, use the session test page from the following thread for this:
showpost.php?p=23826&postcount=2
if it you are still having problems, add the following code for troubleshooting purposes to the login page just after the body tag:
<pre><?php var_dump($_SESSION); ?></pre>
after testing the login again, post back the session var dump results, a copy of the login page and the webassist/securityassist/helpergrouprules.php file.