I got the solution.
When the laptop is in the dock the xserver doesn't recognize the graphic adapter and turns to vesa mode.
The solution is simple: Create an xorg.conf and it works.
It's a little bit like in the 90's when I bought my first Suse 5.2 Distro with a book in a bookstore and wrote my own xserver-configuration to get a graphical desktop. 
I wrote this little xorg.conf and copied it to /usr/lib/X11
Section "Device"
Identifier "Intel Graphics"
Driver "intel"
Option "AccelMethod" "sna"
Option "TearFree" "true"
EndSection
After restart the switching between the tty's works like it should.
I compared the Xorg.0.log before and after adding xorg.conf with Meld, so you can exactly see what was going on.
