Child-menu in Sidebar disappears

Hello,

I have a big problem with my child menu in the sidebar:

First, please take a look here:
https://goo.gl/45WLLF
This is the parent menu which looks and works fine.

Know when I click on the first child also everything looks good as you can see here:
https://goo.gl/MXIc9J

But then when I click on the first grandchild the whole menu disappears and moves down to the left as you can see here:
https://goo.gl/d3LXA9

I can’t figure out why this happens, and I greatly appreciate helpful advices.

Thank you

Karsten