Allow Domain authentication for L2TP VPN
Required Domain authantication for L2TP VPN.
only local users are able to connect through L2TP but not domain users
You can authenticate AD users for L2TP but you have to change the default Authentication to PAP and also in Windows 10 allowed protocols.
I believe the PAP issue is a known issue NC-8891
Use SSL VPN - its better anyway ;-)