Hello
Is there any way to change the sequence of the pages (on top of the screen and in the widget) or is that a WP-question?

Yours
Filip

In Atahualpa at ATO->Page Menu Bar->Sorting order of Page Menu Bar, you can sort by page title or menu_order.
Menu_order is set in the wordpress page edit options

The My Page Order plugin works nicely for this.

tried to download this, but when I clicked on the download, it took me back to the original developer site. I do not use wordpress.org, my site is hosted on a private server.

did you try to download it from the http://wordpress.org/extend/plugins/my-page-order/ page?
I just tried, hit the orange download button and it downloaded for me.