There is a tool to correct Error 720 on Windows 8 & 8.1 VPN (Built-in client). I used this ages ago but lost track of it until a random 8.1 machine with this old issue today.
https://www.vpnhosting.cz/index.php/clanky/wan-miniport-repair-tool-solve-vpn-and-dial-up-error-code-720-and-similar-ppperrors.html
