my problem is explained fast and quick:
Whatever I put as first element in the Navigationbar (via menu) is not clickable. In this case "Startseite" (german for "Home")
It does not work turning all plugins off, but works using TwentyTwelve.
Even more curious: The link works shortly while the page is loading, but then refuses to - it's like something is changing it's property.
Do you have a guess where I should start searching?