Why is using ntrights.exe in Windows Server 2008 R2 giving error openpolicy -1073741790

With UAC enabled elevation is always required however the built in Administrator account (the single account with SID S-1-5-21-xxxxxxx-500) and members of the "Domain Admins" group ( SID: S-1-5-21-domain-512) get automatically elevated, without the need for a human to respond to a prompt interactively. Accounts that are "merely" members of the local Administrators group will not (normally) auto-elevate, leading to the problem that you see.

This default behaviour can be modified by GPO to include other accounts\groups - the details of how to do that are in this technet article