So my promised input to the whole thing:
I gave up on kweb and using midori browser. It automaticly starts in fullscreen and kiosk mode. I have to put in a delay after “node serveronly” of 45s.
Bootup in console mode and add this in /etc/rc.local
sudo -u mirror /home/mirror/MagicMirror/serveronly
sudo -u mirror midori -e Fullscreen -a http://localhost:8080
and thats it. Works for me.