When I enable the "screen keyboard" from the accessibility next to that the GNOME onscreen keyboard is started onboard is also opened. Is there a way to prevent this?!
This a known bug. But if you're planning to use only Gnome Shell instead of Unity you could uninstall onboard
If you do this you won't be able to use onboard on LightDM.
Anyway, it seems possible to use LightDM with Caribou, but the unity-greeter is coded to use Onboard, so it would be necessary to write a patch for unity-greeter.
EDIT. Another workaround is to remove onboard autostart from /etc/xdg/autostart without removing onboard.