I haven't changed the sub nav width, as mentioned in another post here, because then the hover width has not changed with it.
So I have discovered yesterday another way: in the new menu manager of WP 3 I had changed the "appearing name" of the navigation items just in a shorter name. So I need not to change any CSS.
So I have let all as it was.
I discovered with firebug right now, that the the 2nd level items of the last item "TERMINE" got another CSS style than the items of the navigation in front of this last.
The style for the last 2nd level item (here: "Termine" --> "Ausstellungen") has this style:
http://i29.tinypic.com/2lyt5x.jpg (no boxes around 2nd level menu)
and all other 2nd level items, like "Kunstmarketing-Tipps" under "Kunstmarketing" at the left of "Termine", has another style:
http://i32.tinypic.com/qr0wsh.jpg (with boxes around 2nd level menu)
But I have no idea, where in the theme this is assigned to.
ok, it is a theme's problem:
I have installed quickly a brand new Wordpress 3 with NEWSCAST on another server, with no changes to whatever, no CSS changed, no plugins, no contents here:
Look at the grey navigation at "Kategorie F": there the 2nd and 3rd level has boxes.
Then look at the grey navigation at "Kategorie G": there are no boxes around the 2nd and 3rd level of this item.