Hi,
I finally found the reason for this problem, but unfortunately no solution.
The problem was introduced by this commit of Brion: http://svn.wikimedia.org/viewvc/mediawiki/trunk/phase3/includes/OutputPage.p...
Maybe someone of you has a solution. The problem is really bugging me as I have it on serveral servers and I can no longer use the wikis I have once upgraded to trunk (unless I use my dirty hack).