While I was logged in as administrator, I used my husbands computer to check and see what that site looked like and it was fine (but he wasn't signed in) but not on my computer.
It looks like it's likely a problem with trying to embed the login in your content. Try disabling that addon module or plugin in Joomla's backend and see what happens.
Nope, doesn't seem to make a difference. I disabled plugin Content - Load Module, unpublished the article, placed mod CB Login in a mod position and it still gets the page messed up when logged in as anything other than Registered or Author.
So I don't think that it has anything to do with the loadmodule code.
I have the Simple Slide show (bretteleben.de Simple Picture Slideshow) embedded in a table within text. Now it might be a problem with the table but why only when logged in above registered or author.
OK decided to unpublish Simple Slide Show and the article it was in on the front page.
Logged in as administrator and low and behold both the article and the Simple Slide Show where there twice. Not there at all when I log out.