Hi there,
it is odd, as i would expect it to not work on any size screen, as one issue i see is there is more than one instance of an ID. For example in the menu there is: Referenzen which links to the anchor: #ReferenzenThorstenBuch
If i search the HTML of the page for #ReferenzenThorstenBuch i see 1 x instance of it in the Menu which is correct. And 3 x instances of it on the page. When there should only be 1. So i think the page scroll to ID is getting confused.