ok, I have deleted most of my questions, that I had here before, because I have solved them after many hours of try&error.
Upgrading is a horror :-(
Still one question remains:
showing the "BF recent posts" in sidebar: these posts have in sidebar no longer images.
The posts itself do have images, inserted with Wordpress 3.0.1 feature "thumbnail", but "BF Recent Posts" widget in sidebar does not show them (but these images are shown at frontpage for the last 3 posts!!!):
With previous version of Bigfeature and BFnav the "recent post images" in sidebar have been shown!
I use the child theme BFnav 1.0.3 and Bigfeature 188.8.131.52.