Thread:Project:Support desk/Cannot edit Common.css (or any MediaWiki:* page)

I have a wiki up and running (www.makehumancommunity.org/wiki if you're interested). As far as I know everything works as expected, and as the admin I'm able to use all special pages and edit all common pages. Except the ones in the MediaWiki namespace. Specifically, I can't edit MediaWiki:Common.css.

As soon as I navigate to that page, the interface looks as if I'm not logged in: The "log out" link changes to "log in", there is no edit tab etc.

Now, I'm pretty sure I caused this, either with the phpbb authentication plugin or with the plain url settings in apache, the latter which look like this:

RewriteEngine On RewriteCond %{REQUEST_FILENAME} !-f RewriteCond %{REQUEST_FILENAME} !-d RewriteRule ^/?wiki(/[^/]*)?$ /home/joepal/mediawiki/index.php [L] RewriteRule ^/?wiki(/Special.*)?$ /home/joepal/mediawiki/index.php [L] RewriteRule ^/?wiki(/MediaWiki.*)?$ /home/joepal/mediawiki/index.php [L] Alias /wiki /home/joepal/mediawiki

... but, can anyone suggest where/how to start debug this?