The fix ended up being very simple, but not intuitive.
- View your network adaptors by going to Control Panel > Network and Internet > Network Connection
- Right-click on the offending VPN adapter and click Properties
- Click on the Security tab
- Change the "Type of VPN" value from IKEv2 to Point to Point Tunneling Protocol (PPTP)
- Click OK
It seems that when this property is set to Automatic the WAN Miniport defaults to IKEv2 (and gets stuck if this is not the VPN type used).Translated: it is a feature not a bug.
Now attempt to connect again. Short of other issues, it will connect successfully.