Top menu is taking over whole page

I have a problem with “WordPress Menu”.

I add it to the website header, as horizontal orientation.

The problem I have is:

When editing the header and the top menu, it looks like I wish it to be. Also – on pre-view it looks OK.

However, when opening the website on other PC or in incognito mode – the top menu is taking over the whole page in vertical orientation, and the page itself is not shown at all.

I tried to delete the top menu and redefine it – and the problem remains.

I tried to delete the menu, and the web site is shown as it should be, without the top menu.

I tried to add again the top menu, and hide it – in this case the website again looks OK, without the top menu, of course.

How can I fix it?

Thanks

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