Big Problem with menu in WP 4.1

Hello Friends,

I have a problem wit my website (www.spielmannszug-olpe.de) with WordPress 4.1.

The problem ist the following:

If I change something in my existing Menu, (doenst matter what I do, adding a site, removing a side, change the display name of a site…) WordPress kills my menu, deleted the half of the manu, and removes the “Main Menu” Flag for this menu.
If I set thes flag again, the menu (half the muneu, because it has deleted the other half) appears again, but if I change anything the problem appears again as described.

I deleted the whole menu and set it up again (with another name) but at some ponit the problem appears again!

I deactivated all plugins, but that didnt fixed the problem.

I am kind of helpless at the moment.

I hope you can help me!

Greetings
Kai