Hi,
it was not a pending windows update, so a reboot did not solve my problem. Sorry for that, in my initial post I forgot the very important word "not".
One of our microsoft expert found a solution. It was in my personal admin windows profile. He found the solution here: Error 8009001A. It has something to do what the user "J.A. Coutts" wrote in his last post.
The solution is: We deleted my profile and a created it new with a new login and that´s it! Installation resp. update worked now without any problem.
Best regards, Tom