JSI Tip 3424. 'The specified Setup script file (<path>\unattend.txt) is inaccessible or invalid'?

When you attempt an unattended upgrade of Windows NT 4.0 to Windows 2000, you receive:

The specified Setup script file (<path>\unattend.txt) is inaccessible or invalid.
Contact your system administrator.

NOTE: This error occurs with winnt32 /unattend:unattend.txt.

You are missing the UnattendMode=FullUnattended parameter in the unattend.txt file:

   \[Unattended\]
   NTUpgrade=Yes
   UnattendMode=FullUnattended

   \[UserData\]
   ProductID="xxxxx-xxxxx-xxxxx-xxxxx-xxxxx"