For the record: I have moved away ~/.kde and the problem disappeared. I guess some old state in there caused the problem. Obviously KDE5 does not make much use of it anymore as I lost only some settings...
Bug#793824: plasma-nm: plasma-widget-networkmanager has totally disappeared with plasma-nm 4:5.3.2-1
- Bug#793824: plasma-nm: plasma-widget-networkmanager has total... Jan Klötzke
- Bug#793824: plasma-nm: plasma-widget-networkmanager has ... Jan Klötzke