ORA-12640 aka TNS-12640

I recently installed patch 10.1.0.5 on my development box, a windows 2000 box.
When i install oracle software i always log on with the OS administrator account, but when i am developing applications, i normally log on as a power user because i don't need administrator privileges.
For some reason that i still have to investigate, after patching the db, i could no longer log on to the db from the command line using something like:

sqlplus /nolog

connect / as sysdba

because the following error was returned:

ORA-12640: Authentication adapter initialization failed

After a few attempts, it turned out that if i added the os user to the administrators group, the same log on procedure would succeed.
To be more precise i had also to sign off and on again in order to acknowledge the change of permissions.

This behaviour seems to be related to the protocol that oracle uses to communicate with the db when the client and the server are located on the same box.
Indeed it is remarkable to note that if you try to log on in this way:

sqlplus "usr/pwd@dbconn as sysdba"

you'll get the same ORA-12640 error if you do it on the server, while it would succeed on a remote client.

And the same happens when running Golden32.exe or PLEdit.exe , two nifty utilities from Benthic Software.

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ORA-12640: Inizializzazione adattatore di autenticazione non riuscita
ORA-12640: Fallo en la inicialización del adaptador de autenticación
ORA-12640: No s'ha pogut inicialitzar l'adaptador d'autenticació
ORA-12640: Initialisation impossible de l'adaptateur d'authentification
ORA-12640: Inititalisierung des Berechtigungsprüfungsadapters misslungen
ORA-12640: Αρχικοποίηση οδηγού προσαρμογής ελέγχου ταυτότητας απέτυχε
ORA-12640: Initialisering af autentificeringsadapter fejlede
ORA-12640: Fel vid initiering av adapter för autentisering
ORA-12640: Initialisering av autentiseringsadapter mislyktes
ORA-12640: Valtuustarkistussovittimen alustus epäonnistui
ORA-12640: Az igazolási szolgáltatás csatolójának inicializálása sikertelen
ORA-12640: Iniţializarea adaptorului pentru autentificare a eşuat
ORA-12640: Initialiseren van verificatieadapter is mislukt.
ORA-12640: Falha ao inicializar o adaptador de autenticação
ORA-12640: Falha na inicialização do adaptador de autenticação
ORA-12640: Сбой при инициализации адаптера аутентификации
ORA-12640: Selhala inicializace ověřovacího adaptéru
ORA-12640: Iniciácia autentifikačného adaptéra zlyhala
ORA-12640: Niepowodzenie inicjalizacji adaptera identyfikacji
ORA-12640: Yetki doğrulama adaptörü başlatılamadı