No users in Local Administrators Group

[am]

When attempting to view the members of the local administrators group I see
no users at all, even when in the context of the local administrators
account. If I attempt to add an account to the group that I know is in there
already, I receive the following message:

"Username" is already a member of group "Administrators".

I've also tried running a script to enumerate the users in local
administrators group and this returns no results.

Any ideas how I can resolve this?

 

[am]

I knocked up a C# app that calls NetLocalGroupGetMembers but this returns 87
(ERROR_INVALID_PARAMETER) when the groupname parameter = "administrators".
When groupname = "users" the function returns 0 (ERROR_SUCCESS), indicating
that it is succesful.

 

[am]

I've done some further debugging of the NetLocalGroupGetMembers API and looks
like it calls LsarLookupSids2 (translates SIDS into names), which fails -
returning C000000D (STATUS_INVALID_PARAMETER in ntstatus.h). This then gets
translated to 87 (ERROR_INVALID_PARAMETER in winerror.h) before being
returned by NetLocalGroupGetMembers.

Running "net localgroup administrators" returns :

Alias name administrators
Comment Administrators have complete and unrestricted access to the
computer/domain
System error 87 has occurred.

The parameter is incorrect.