[Theme: Twenty Fourteen] Using grid for featured content in category template

Hi,
I am trying to have an overview (like the featured content) at the top of the catergory page:
https://unterwegs.frank-müller.net/reise-nach/iran/

I added a loop that refers to a copy of the content-featured-content.php and added grid to the body_class(‘grid’).

For some reason, the first row shows only two items instead of three
every other row is fine (with three).

On the homepage it is fine (with three) as well.
The only difference I see is the
<div id=”main-content” class=”main-content”>
If I add this to the category page everything is black ??
Does anyone have an idea, why there is this gap?
Thank you very much
Frank J.