EASY FIX!
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one potential cause of this is whitespace at the end of wp-config.php
edit this file and delete any whitespace from beyond the ?>
Someone please alert the proper authorities.
It’s a little complicated jbiziou, Try installing a different theme, it’s probably easier than widening the default one.
oh, and there’s another one
http://jp.jixor.com/plugins/static-page-extended/
that uses external files as frontpages or some such.
there’s a static pages plugin i read about in the wiki
http://codex.wordpress.org/Pages
it points to this
http://www.semiologic.com/projects/static-front/
This should allow you to have whatever you wish upfront.
hope that helps