Could it be that one of your plugin is rewriting the URLs? It's very strange and if you have updated your store by clicking the [Update Settings Only] button, I'm stumped.
When I look at the source code of that link it has changed from this:
<a href="[product.url direct='1']" class="dfbutton dforange dflarge" title="Buy this product">Kaufen</a>
<a rel="external nofollow" title="" target="_self" href="http://www.omasgeheimnisse.com/homepage/5wYU">Kaufen</a>
I think there must be another plugin affecting this link.