VPN server authentication error
The article that you've referenced seems to mention that the fix for the CHAP authentication in newer systems is to modify the file at /Library/Preferences/SystemConfiguration/com.apple.RemoteAccessServers.plist by changing the value of "AuthenticatorProtocol" from "MSCHAPv2" to "PAP":

AuthenticatorProtocol = (PAP);


If you still have issues, posting the contents of /var/log/ppp/vpnd.log may also be helpful.

I do not recommend using the dscl command that you're referencing above to configure your user record. The 'pwpolicy' command could be better suited for that task.