I got this excellent tip from a user in our forum. It would come in handy if you just upgrades WordPress and are getting this error:
Now, that 100 million sites are upgrading to WordPress 3.0, there’s a good chance folks get bitten by it.
If your WordPress install is a non-English version, and you haven’t changed the default authorization keys in wp-config.php to something else, it’s possible that the WPML install will fail and prevent you from logging into the admin area with a message in your language with a meaning similar to ‘You do not have sufficient permissions to access this page.’
To prevent this issue and be able to install WPML without problem, please first change the four authorization keys on your wp-config.php to something else. You can use the following link to create a set of four random keys ready to be pasted into your wp-config.php:
Changing these keys will cause all logged in users to be logged out, forcing them to log in again the next time they access your blog. Other than that, there are no ill effect.
If you installed WPML without taking the above precaution and are now unable to access your admin area, delete the WPML folder, sitepress-multilingual-cms, from within wp-content/plugins, and you’ll be able to login into it again. Then follow the above procedure to be able to install it.
This behavior is caused by a bug in the WordPress core that can also affect other plugins. Changing the authorization keys from their default value is the safest way to avoid potential problems not only with WPML, but with them too.