the first thing I would suspect is that there is an issue on the server preventing sessions from being maintained.
test the servers session management using the sessionTest.php page from the following thread:
if the session test fails, you will need to contact the host to have them correct the problem, it is most likely an issue with the session.save_path in the php.ini file.
If the session test passes, the try adding the following code to the login page, after the body tag to help troubleshoot:
<pre><?php var_dump($_SESSION); ?></pre>
this will write the session contents to the page to help troubleshoot.
post back the session contents after trying to login and a copy of the visitors_Profile.php page and the WA_SecurityAssist/HelperGroupRules_PHP.php file.