Administrator user account not giving administration rights?

I have a Mac mini server for which I forgot the Administrator password so I have followed several of the guides that talk me through creating a new administrator level account from the command line during the boot process. I've even used the hidden password reset utility to change all user passwords (to the same phrase), however, none of these allow me to subsequently go into the users and groups section of system preferences and unlock / manage any of the accounts.

Out of sheer desperation I have used another of the command line boot options to re-install OS X and I now have version 10.7.5 as well as server 10.7.5 (1.5.0) however for the life of me I still cannot get Administrator privileges to work with any user account, nor can I successfully log in to the server application.

It used to work fine until an automatic update (to 10.7.4?) several years ago and since that time it has laid dormant until the last day or so.

Can anybody out there help me to get a working system once again, even if I have to start from scratch and lose all content on the server?