Is it possible the main.php page is redirecting them to the other page? What is the other page? There was an issue in previous versions that it was ignoring the setting:
$Authenticate->AutoReturn = false;
and returning people to the last restricted page they tried to visit even when it was turned off. That should be fixed in the latest version of authenication.php.... I've attached it so you can try that. If that fixes it, reinstall the latest version of the extension so the new version is used in future applications by default.