Already tryed the shell with 'sudo apt-get update' . It shows:
...
OK:1 http://graphics.tuxedocomputers.com/ubuntu xenial InRelease
OK:2 http://ppa.launchpad.net/elementary-os/stable/ubuntu xenial InRelease
... and so on: OK
...
But still two updates are in line in the Appcenter. One of it is "Betriebssystem".
Any ideas?