Docker hangs on VPN

I use DockerToolbox on Windows 7, and it usually works fine. But when I log into my corporate VPN, Docker hangs. Disconnecting from VPN makes Docker functional again.

I believe a blocked port is causing the docker daemon to not run. Does anyone know what ports need to be opened? I have already opened ports 2375, 2376, but it did not make a difference.

Could there be anything else causing this problem?