I checked "settings" table and found, that lot of settings did only have a php-variable but no value. See screenshot.
Furthermore the old "dark-borders" were set as theme and not "vB
That's it - all is fine. I even can go indadmin panel because the cookie value was a variable too.
I think I did all nessessary upgrade steps, but I don't know, what was missing.
Ahhhh, the adminlog table was not renamed to admlog. So something went wrong, but now is fine.
If anybody has same problems: check "settings" table with phpmyadmin...
Thanks a lot Chuck for beeing with me!