Just a quick note on a fix if you happen to be experiencing a problem logging into a Drupal site.
The behavior is the login seems to silently fail. No error message, no log messages, nothing. Login/password is accepted (and bad passwords will fail - so it is hitting the db properly), but after submitting you are returned to the front page not logged in.
I solved the issue by checking the cookie_domain setting in the settings.php file. The domain name is parsed from the $_SERVER['HPPT_HOST'] php variable using some regex. If this is done incorrectly, the wrong domain will be set in the php session.cookie_domain variable which will prevent logins. To fix, you can hard set the $domain variable in the settings.php file. There are notes on this in the settings.php file, but it is a little obscure.