I am sorry, but I don't understand what do you mean here. Facebook actually requires you to be logged in before allowing you to post anything.

I'm logged in as superuser. But look what happens when I want to share this post on Facebook, which will then be displayed on Facebook. It is the "login" and not the contribution.
With Google+ but it works correctly

Do you require users to be logged into your site to view your blog post? If that is the case, this is the correct behavior because when you share a URL to Facebook, they will need to crawl your site to retrieve the contents.

I believe the contents is cached. I have placed the URL at https://developers.facebook.com/tools/debug/sharing/?q=https%3A%2F%2Fwww.schwedenhaus.ag%2Fblog%2Feinfach-ein-eintrag-mit-text-und-bild-und-weiterleiten and it's showing the correct content now.