ORA-12505 trying to connect to a remote database

Hi,

I want to connect to a remote database (database version : XY).

Here is the tnsnames.ora on the client :

AB =
  (DESCRIPTION =
    (ADDRESS_LIST =
      (ADDRESS = (PROTOCOL = TCP)(HOST = xxx.xx.xxx.yyy)(PORT = 1521))
    )
    (CONNECT_DATA =
      (SID = XY)
      (SERVER = DEDICATED)
    )
  )

The tnsping works :

\rtrebuch>c:\oracle\ora10g\bin\tnsping AB

TNS Ping Utility for 32-bit Windows: Version 10.2.0.1.0 - Production on 28-AVR.
-2009 14:10:31

Copyright (c) 1997, 2005, Oracle.  All rights reserved.

Fichiers de paramÞtres utilisÚs :
C:\oracle\ora10g\network\admin\sqlnet.ora

Adaptateur TNSNAMES utilisÚ pour la rÚsolution de l'alias
Attempting to contact (DESCRIPTION = (ADDRESS_LIST = (ADDRESS = (PROTOCOL = TCP)
(HOST = xxx.xx.xxx.yyy)(PORT = 1521))) (CONNECT_DATA = (SID = XY) (SERVER = DEDICATED)))