Menu Trouble

I am the owner of systemics-academy.com

It is multilingual (German and English). I need to edit the English-language main menu (independently of the German one). Up until recently, I have had no problems with that.

But now, I go to dashboard –> apearance –> menus, then select “main menu” and edit it (move the menu tabs about and create sub-menues etc). I then press “save menu” and get the error message “page not found”.

What’s the problem and how can I solve it?

Your support is much appreciated !!

Greetings from Berlin,
Ben

The page I need help with: [log in to see the link]