To incursio et al:
I just spent several hours working this as well! After I double checked everything, reread this forum and all related links – I STILL COULDN’T GET IT TO WORK.
I’m not an idiot – so clearly I was missing something obvious because other people got it to work. So, I started to debug it.
Then I found out… This line in wp-config.php
define( 'WP_CACHE', true );
must come before this line:
require_once(ABSPATH.'wp-settings.php');
Hope that helps…