Now you must put this pieces together
Starting from FORFILES ending with /norestart"
The Forum didn't allow to post the complete command - Maybe due to security reasons!
So I used the command for Explorer version 10, 9 and 8.
Just replace
-*11.*.
by
-*10.*.
-*9.*.
-*8.*.
In a row. Another restart of the PC.
After that the updater started directly to update the firmware without the serial number popup.
Maybe because I filled it out in previous trials.
Thanks AVID SUPPORT! Finally I got my Eleven Rack upgraded!