As i was very glad finding a plugin with that functionality, i found out that, after the blog being transferred from the development- to the live-domain, the source code had following insert, which made a lot of problems, as you can imagine:
<!-- Dynamic Widgets v1.5.4 by QURL - http://www.qurl.nl //-->
position: absolute !important;
clip: rect(1px 1px 1px 1px); /* IE7 */
clip: rect(1px, 1px, 1px, 1px);
I didn't manage to delete it, even not be deleting the plugin. What is this for and why is it written directly into the source code?