In its development tools console (F12, console tab, refresh page) IE 9 is complaining about this (from the page's source code):
Now it's possible that your server is adding that automatically. If so you have to contact them as to how to change that to a secure protocol. However, if it is in the source code as you have it, it needs to be changed.
<p class="akt"> </p>
<!-- GOOGLE ANALYTICS -->
And the code in that script writes code to the page that includes an image with an http reference. If changing the script's reference to https doesn't change the image's reference to https, it will still cause a problem.
If it's something you're adding you could just get rid of it. It looks third party though, so perhaps just changing the protocol will fix it. If it's something your host is adding, you will have to ask them how to change the protocol on the script and if that doesn't change it on the image as well, then you have to ask them how to change that.
I also noticed this which IE 9 is not mentioning:
That might need to be changed as well. If there is no https version of that page, the tag can safely be dropped.
<meta name="description" content="Jetzt ★★★★★-Qualität bei Petico.de: Flockenmischfutter für ausgewachsene kleine und mittelgroße Hunde (von 5 bis 25 kg Gewicht) mit normaler Aktivität. Royal Canin Special Club Performance Adult Trad 15 KG VORRATSPACKUNG. - In dieser Kategorie finden Sie Hundefutter von Royal Canin." />
<meta name="robots" content="index, follow" />
<link rel="canonical" href="http://www.petico.de/royal-canin-special-club-performance-adult-trad.php" />
<link rel="stylesheet" type="text/css" href="collmex.css" />