I found a solution for the problem that new faces are not detected anymore in Organizer 11. Seems that others have the same problem and it is a bug in the Organizer 11.
This solution fixed it also on my PC:
Now I have only the question why the import of pictures with face tags does not work correctly as described above. I don't need it currently, but exactly this is the background why I store the keywords inside the *.jpg files, that I can reimport them later if a catalog is damaged or I get pictures from friends which are already tagged.
But to make this working Organizer need a function to assign imported keywords to person/face tags that the pictures can be filtered similar to detected faces by the Organizer itself.
Hope that someone know a solution :-)