Spreadshirt shop integration problem on website

Hello,

I have a problem with the integration of our Spreadshirt shop on our website (integrated via official embed code). When you click on an item, which you would like to buy and then want go to your shopping cart, that part is loaded behind the other content. There is another problem with the "Impressum" page (top and buttom part is cut out), which is important for a German shop.

The Spreadshirt Shop itself works, when not visited via our website (can be integrated with an embed code)

Spreadshirt shop (WORKS CORRECT): https://shop.spreadshirt.de/querverstand/
Shop on our Website (PROBLEMS): https://querverstand.de/shop/

Do you know what could be the reason? I think it has something to do with Javascript but I am not sure, as I am not a developer.

Embed Code (copied from Spreadshirt on our shop admin section):

<div id="myShop"><a href="//shop.spreadshirt.de/querverstand">querverstand</a></div>
<script>
    var spread_shop_config = {
        shopName: 'querverstand',
        locale: 'de_DE',
        prefix: '//shop.spreadshirt.de',
        baseId: 'myShop'
    };
</script>

<script type="text/javascript" src="//shop.spreadshirt.de/shopfiles/shopclient/shopclient.nocache.js">
</script>