Thank you very much for your replies. I tried fixing it in KDE 3.2, but that didn't work: Kontact did not remember the RDF-feeds added to the summary and only appointments were displayed.
After updating to KDE 3.2.1, however, everything worked perfectly. (Displaying the KOrganizer is not implemented, but now that this works I don't really need it any more.)
Thanks again,
CML