I have not done NT setup for a long time. I do not remmeber something in detail.
I think your NT must be a NT Server and you did not selected your NT Server as the primary domain controller in the domain "Mydomain" when you installed your NT Server.
Another way for you to solve this problem is to intall your NT as a NT Workstation.
If you have tried what I suggested above, please let me know the result, so I can know if the memory chips or harddisks in my brain still work well. Thank you!