I've found the bug by myself... It is, because the Worklist WebPart creates the following URLs:
http://dev4:101/RuntimeServices/OpenInfoPathTask.aspx?K2Server=dev4&SN=140_19&XmlFN=Invoicepublished1&Lib=http://dev4:901/Invoicing&OpenAsWebPage=True&SPURL=http://dev4:901&Single=False
But the "Lib=" string has to be relative and not absolute! So it seems, that I have to create a working task list webpart by myself...
www.avanade.com
http://blog.mwiedemeyer.de