Sorry - I am not sure what else to suggest. I compared your .vbox file with my running Sierra VM and the only real differences were the video resolution (my VM is 1920x1080) and the USB interface - I am using XHCI
One other thing you might try is to disconnect the host from the internet (i.e. unplug ethernet and stop wi-fi) and then manually set the clock to 1 Jan 2019. A lot of certificates expired a couple of years ago, although I seem to remember the problem didn't manifest until the installer displayed its GUI. See: viewtopic.php?p=475887#p475887