[10.2.0.1 onW2K3] error ORA-12638 when connecting

Hi, Today suddenly every new connexion to one of our applications failed.
The error ORA-12638 was displayed and the following lines in sqlnet.log :

***********************************************************************
Fatal NI connect error 12638, connecting to:
 (LOCAL=NO)

  VERSION INFORMATION:
    TNS for 32-bit Windows: Version 10.2.0.1.0 - Production
    Oracle Bequeath NT Protocol Adapter for 32-bit Windows: Version 10.2.0.1.0 - Production
    Windows NT TCP/IP NT Protocol Adapter for 32-bit Windows: Version 10.2.0.1.0 - Production
  Time: 23-NOV. -2010 11:09:44
  Tracing not turned on.
  Tns error struct:
    ns main err code: 12638
    TNS-12638: Echec de recherche de références
    ns secondary err code: 0
    nt main err code: 0
    nt secondary err code: 0
    nt OS err code: 0