I have renamed plasma-desktop-appletsrc and plasma-desktop doesn't crash anymore. I re-created my whole desktop layouts and it still does not crash. What I did not add are the rss/news widgets and my todo widget...
The CPU usage now is 10% higher than in my former working configuration...
Other issue now: the shelves in the panel don't work anymore. They can e.g. show the favorites, but they cannot show any application category. When I choose an application category the tooltip of the shelf stays blank and the shelf is empty...