Customize header menu

http://www.algonuevo.de

that’s my site and I can not customize the header menu anymore.
The footer is no problem.

The header-menu though, which is the main navigation in my site, is generated according to the order of my MAIN pages (I have to set individual pages to “private” if I wanted to exclude them from the menu, which is, of course, a silly workaround.

1. It is not possible (under “appearance” – “menus”) to add pages manually to a fresh menu (my customized header menau). It worked ALL the TIME!

I created a NEW header menu, and selected it as my header navigation menu in “customization”.

And this is what I get, when I “add pages to the menu”:

“Drag each item into the order you prefer. Click the arrow on the right of the item to reveal additional configuration options.”

There are no “items” anymore (those boxes, that you can hierarchically drag around to construct the menu).

I am using the latest version of firefox and the latest release of decode together with WP 4.1