I just found some Information on the Internet.
The cause of the Problem appears to be an issue between Network Adapters and Cisco VPN Client 5.0.07.0440, which I had installed. The solution seems to be to roll back to Windows 8.1, uninstall the Cisco VPN Client and then do the upgrade again.
I will try this now and post the results afterwards.
Thanks for the information, let us know your results.
It worked indeed!