In which browser on what computer you see this line? We did checked on 5 different computers with same result and there is no lines under "Referenzen" menu item. It looks exactly the same as on screenshots above.
Sorry, for the whole time we thought that this is the only page where you don't have it. We checked it closer and we found that this line exist there. You just don't see it because of the header background what you can see on http://pasteboard.co/2FTuIPHC.png
It does not matter if it's portfolio page or not. Did you read the article of wordpress menu we gave you above? There are all information about how to make the right hierarchy for pages.
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Comments
We suggest to check hierarchy under Pages > All pages section if parent/children connections are ok.
For more details about menus and it's structures, we suggest to read https://codex.wordpress.org/WordPress_Menu_User_Guide
Thanks!