Failed installing RAD studio 10.1 Berlin.

Hi, I have problems installing RAD studio 10.1 Berlin on a Windows10Pro PC (64Bit) . The installation failes in an early with the message "Failed installing Microsoft .NET framework 2.5. An active internet connection is required.". BUT the my internet connection is fine. The PC has a fresh installation with Windows10. Under Windows7 before RAD-Studio worked without problems Any ideas ?   Thanks Klaus