Macbook Bootcamp Windows 10 WiFi issues
Deinstalling Cisco VPN Client fixed my problem. 

There seems to be a problem with updating from Windows 8 to Windows 10 when Cisco VPN Client is installed (https://superuser.com/questions/947651/no-network-connection-after-upgrading-to-windows-10). 

Although my Windows 10 was a fresh install, deinstalling Cisco VPN Client helped me getting a stable internet connection.