Assigning IPv6 addresses with VirtualBox provider on Windows host fails

I tried to use an IPv6 address with Vagrant, but starting the box fails.

Vagrant version

Vagrant 2.1.2 VirtualBox 5.2.18

Host operating system

Windows 7 64-bit, Windows 10 64-bit

Guest operating system

Ubuntu Bionic 64-bit

Vagrantfile

Vagrant.configure("2") do |config|
  config.vm.box = "ubuntu/bionic64"
  config.vm.network "private_network", ip: "fd12:3456:789a:1::2"
end

Debug output

==> default: Clearing any previously set network interfaces...
There was an error while executing `VBoxManage`, a CLI used by Vagrant
for controlling VirtualBox. The command and stderr is shown below.

Command: ["hostonlyif", "ipconfig", "VirtualBox Host-Only Ethernet Adapter #3", "--ipv6", "fd12:3456:789a:1::1", "--netmasklengthv6", "64"]

Stderr: VBoxManage.exe: error: IPv6 setting is not supported for this adapter

Expected behavior

The IPv6 address should be assigned to the Host-Only adapter in VirtualBox, and as well inside the host operating system, and the startup process should continue.

Actual behavior

The startup process fails

Steps to reproduce

1. vagrant up.

VirtualBox seems to not be able to configure IPv6 addresses properly on Windows, the input boxes are greyed out in the configuration dialog (see https://imgur.com/htJOTqr). I can configure the address manually using Windows network adapter settings.