I have to check on that, but I think this error can be caused, if you installed wordpress2 and did not only do an upgrade.
From the Readme:
1. Delete your old WP files, saving ones you’ve modified
2. Upload the new files
3. Point your browser to /wp-admin/upgrade.php
4. You wanted more, perhaps? That’s it!
There’s nothing about doing an install. If you did, it creates the admin user (UID 1) and thus your old user is gone and the password is random.
Just my idea what could cause this. As I said, this must be approved.
– David (xy77)
