No CSS anymore!
That’s frustrating. Looking with my browser tools, I see, that the load.php from mediawiki returns nothing. Searching around and found only one place which helped:
https://bugs.php.net/bug.php?id=64836 <– the bug entry on php.
Now I tried to downgrade my gentoo package sys-apps/file back to 5.11 and restarting my apache … viola … it works.
Now the load.php works again and it looks like, that this has nothing to do with mediawiki itself.