I have fiddled around with PyQt again and - exactly it doesn't work... The funny thing is that Scribus doesn't crash - it closes.
Some time ago I read in a book about python gui-toolkits that you can only have one instance of QApplication and that this would even go for Qt-Apps that use python as a scripting language. ?Does that mean that you have to somehow spawn off new windows from the running app? how the hack would this work? Apparently it is not a Sip problem but rather something you have to live with if you are using Qt.
