Unfortunatlly that was the only one server on which I could delete the problem computer groups.
I have tried with the same procedure on other five downstream servers due to delete the false computer groups, but it does not work. That groups can not be deleted. And there is no updates approved for these groups anymore.
Well as we see WSUS upstream server is not able to just overwrite the DB onto his downstream servers.
It seems now I really have to rebuild all rest 17 replica servers one by one.
kailar