using Google Webmaster tools I saw that I have a lot of crawl errors.
As it turns out the plugin ArticleSuggestions is causing them.
In the tab the avatar is not displayed but although not displayed a href is created:
<div class="as-avatar"><a href="/Guest" ></a></div>
this href is causing a lot of crawl errors because it is pointing to a non-existing url. This error (might) impact my google ranking.
hope this helps!