The problem is that one of your books has an invalid date in it and its causing the program to fail. You could run the plugin in debug mode (see the first post in the plugin's thread - linked in my signature) to find the book.
However, I'll update the plugin to handle this error more gracefully and post it when done (it will show up as an available plugin update in calibre).
I see you originally posted in the German language forum, so I wonder if the book causing the issue has a valid date for the language of the book, but it isn't being read correctly. It would be interesting to see the book.
Last edited by meme; 10-08-2011 at 06:56 AM.