Adobe Flash Player installation failed Background
When the Adobe Flash player update was auto-updated, I had to quit Safari and then continue the installation.
Accidentally, I started Safari again right during the installation procedure.
This resulted in a never-ending install that I had to force-quit.
So, I downloaded the Uninstall Flash Player software to completely remove flash from my system and then re-install it.
Problem
The (new) install failes with the folloing error report:

How can I install flash now? How can this error occur even though Flash was apparently completely removed from the system?
A restart fixed the problem. The installation worked without interruptions.