That's what I googled after I just ran into that problem upgrading my Wordpress instalation using the WPAU plugin.
As the title mentions, after finishing the process, all I got was blank pages to every URL I asked for :/ .
Using Firebug I got to see that it wasn't just empty content, but I was getting a 500 error!, so I checked the HTTP server's error log and ran into this line:
2008-11-20 15:57:14: (mod_fastcgi.c.2592) FastCGI-stderr: PHP Fatal error: Call to undefined function force_ssl_admin() in wp-settings.php on line 394
This meant that during the automatic upgrade process not all the files were automatically upgraded :roll: .
So what I had to do is copy manually all the php files in the root directory, wp-admin and wp-includes, taking care of my wp-config.php and wp-settings.php files :) .
And now we seem to be fully functional again :) .