ConnectEx tcp: No connection could be made because the target machine actively refused it..
Re-installing VirtualBox and Docker didn't help - the setup worked exactly once,but after the next reboot, the dreaded error message appeared again.
The root cause was my corporate VPN client; whenever I establish a VPN connection, the client seems to cut off all other NICs, thereby disabling connections to the DockerHost VirtualBox VM.
So right now, I can either work offline with Docker or work online without Docker. That kind of sucks :-/