If a category has sub categories the page /course/category.php of the parent category breaks XHTML strict.
By changing the row 225 of the file /course/category.php ($Id: category.php,v 1.93.2.9 2007/04/26 08:48:07 moodler Exp $) from
echo '<table align="center" border="0" cellspacing="2" cellpadding="4" class="generalbox">';
to
echo '<table border="0" cellspacing="2" cellpadding="4" class="generalbox boxaligncenter">';
the problem is solved.