WPMU: You do not have sufficient permissions to access this page.

While moving, a blog from WPMU to a single wordpress, I kept getting this message: "You do not have sufficient permissions to access this page." I have played for about an hour with wp_usermeta and wp_users. Tested all kinds of settings in wp_capabilities and wp_user_level but still nothing worked. Then I have spotted it in wp_options. The option_name wp_user_roles was missing and instead was something like wp_123_user_roles. After I renamed it, all worked like a charm.

WordPress.Org and system users

If you notice you have these two strange users in your Wordpress: WordPress.Org, system - then it's very clear your site was compromised and you need to take some steps:

1) Remove those users, preferably manually with phpmyadmin
2) Check your template and your posts for modifications or hidden links insertions
3) Update Wordpress to the latest version
4) Look for backdoors in your .php files, generally malicious stuff is encoded, so check for "eval (" calls.

The server www.xxxxxxx.com at Magic requires a username and password.

Recently one of my wordpress site was giving this message: The server www.xxxxxxx.com at Magic requires a username and password when I was trying to update or preview an old post.

After digging into this for a bit, I found that some files have been modified: ./wp-includes/vars.php and a couple of files in plugins directory like ./wp-content/plugins/akismet/akismet.php.

All these files have lines (usually first line) containing:
eval(gzinflate(base64_decode('1VVtT9swEP7 [...]

After removing these lines (manually via SSH or FTP), the wordpress site turned out to normal.
It seems like one of the wordpress administrator had some trojans on his computer, probably one of them modified the wordpress files through wordpress admin area after he logged in.

You can read more about people complaining about this issue here: http://wordpress.org/support/topic/295482?replies=44