the "Problem" with the splash screen was easily fixed, since only plymouth-theme-ubuntu-text got installed during the upgrade and not plymouth-theme-ubuntu-logo.
First I removed nvidia again and deleted the /etc/X11/xorg.conf.
After a rebooting the brightness keys worked fine (the other behaved strage but I didnt have time to test them).
Then I got back to the newer Bios 11ST, since 09ST has some small problems (dmesg reports hardware issues and videos and sounds stutter and are bucking). Then I noticed that the brightness keys do not work from the end of grub=the beginning of the kernel until gdm. Then (I suppose when your brightness module gets loaded) everything is fine with the brightness-module installed and everything stays bad without it.
Since we now know that its related to nvidia I suggest its out of topic
Sadly I need the nvidia driver, since I am developing on openGL ...
Thank you very much for your time and work,