When using SSTP VPN with MSCHAP2 with "Automatic use credentials" the dialog window asking user for credentials is only a ghost.
I confirm that I can type any string as password and VPN is connecting fine.
( RASDIAL "vpnname" command work without this problem)
Notice: In Control Panel\Internet Options\Connections<VPN name>\Settings\Phone line settings\ username string is always injected by Windows - maybe it bothers?