Getting rid of the Docker error "ConnectEx tcp: No connection could be made because the target machine actively refused it.."

Out of the blue, the Docker setup on my company-provided Windows laptop stopped working; I could launch the "Docker Quickstart Terminal" just fine, but every attempt to run a docker command (e.g. "docker ps") resulted in the error message:

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 :-/