3 of my Users had Administrator rights.  However, the Administrator rights disappeared, and I can now add no User to the Administrator group.  Administrator is not an option when I try to add a User to a group.

 

I can add a User to the Administrators group (plural), and I can still logon to the server as the Administrator.

 

What happened to the Administrator Group and how can I fix the problem?  Any help will be appreciated.