thank you for your reply. The reason was really the missing .NET.
As far as I remember, in this case .NET is installed during the QV-installation. But not this time, you need to preinstall it as a feature. This was new to me, I think its a 2008-Server-Behavior. Nevertheless .NET 3.51 has been installed everything seems to work fine. The service account is like mine member of the local and the Qv-admins.
Thank you again for your help and next time I'll check for .NET first.
The behavior with .NET 3.5 is related to 2008 R2 where you need to add it as a feature. I belive it's yet another step of securing 2008 which is new in R2. I will have a chat with the packaging-crew to see if there's anything we can do in the installation program for future releases, but my guess is that we're not allowed to install .NET 3.5 automatically on 2008 R2