Hi vettenut. Welcome to SitePoint.
The reason for the apparent menu shift is that on this page (unlike all the others) is not taller than the browser window, and thus a side scroller does not appear. Since that does not appear, the page shifts a little as it disappears.
This is natural behavior, and is common, so it's not a problem as such. If you want to stop it, you have a couple of choices. Either add some content to that page, or you can force the page to be at least as tall as the browser window.