In WCAG 2.0 there is a Guideline 2.1 Keyboard Accessible: Make all functionality available from a keyboard.
http://www.w3.org/TR/WCAG20/#keyboard-operation

The main menu does not meet this guideline.

In my frontend I changed in the file include/menu.inc.php this line:

$mmenu_entry = new CCol($menu['label'], $menu_class);

into:

$mmenu_link = new CLink($menu['label'], $menu_url, 'highlight nowrap');
$mmenu_entry = new CCol($mmenu_link, $menu_class);

This solved the problem.