LDAP Authentication is not the same thing as Windows Authentication. LDAP authentication in mojoPortal validates the user against an ldap or active directory server but still uses forms authentication. The user does use their windows username and password with ldap/ad but it is not the same thing as windows authentication. Windows authentication is generally only used on intranets and local networks.
If you want to continue using ldap authentication on your new server you need to get your new server talking to your ldap server. Probably only specific ip addresses are allowed to talk to your ldap or Active Directory machine(s) so you need to check with your ldap or network administrator to allow your new server to connect.
Otherwise you could switch to database authentication by setting useldap to false or 0 in mp_Sites, then recycle the app to clear the site settings cache. But you may have to recover the user's database password since it was not used with ldap and the user does not know what it is.
Hope that helps,