Thank you very much for your help Kaspars, I really appreciate it. Selecting a title solves the problem that no box showed up, but I still have the problem that no content is displayed. You can see that for yourself on my website now, I´ll let the widget online for a while.
I get following error btw: jQuery.easing[jQuery.easing.def] is not a function
source: http://www.boystalk.de/wp-content/plugins/tabbed-widgets/js/jquery.easing.js?ver=2.7.1
line: 46
Michael