[Samba] Win10 - can't create domain user

[samba1 at nym.hush.com]

I need to add a Windows 10 PC to a Samba 4 NT domain.  I know it's not
really a supported option, but I'm using an older build (1607) of
Windows 10 which I believe should work.  
I've done what I would normally do to configure a Windows7 PC
(including the LanmanWorkstation Parameters registry entries), and the
Windows 10 PC seems to have joined the domain (I got the "Welcome to
the domain" message).
However, when I try to add a domain user on the PC, I get a message
saying "This PC is having problems communicating with the domain.  Try
again or contact your domain administrator".
When adding a domain user there's the option to Browse the domain for
users, and when I use that it doesn't show the domain users.  When I
click the "Advanced" button then "Find Now" it prompts for the domain
admin username and password.  When I enter the credentials it shows
all the domain users, but even if I select the user I want to add, I
still get the above error message when I click OK to add the user.
I've checked that SMB 1.0 is installed in Windows 10 (It's the only
SMB version listed in the Features of this build of Windows 10), and
I've also tried setting the Samba documented policy settings for the
UNC Hardened Paths.  I've also added a simple entry in the Windows 10
hosts file for the server, using the format:-
    Local-IP-Address  ServerName
   For example:-   192.168.0.1 sambaserver 
What do you think might be causing the problem?  I'm not sure if the
PC has correctly joined the domain (even though I got the welcome
screen), or if I need to set some additional parameters etc.