PL/SQL :: ORA-29283 Invalid File Operation On Windows System

Windows 2003, DB 10.2.0.3...I have to read some files from remote folder, so on remote Windows machine I shared folder c:est (grant Everyone first, and Administrator then) and on DB Server I mapped the remote folder with letter T..All these operations were made with administrator privileges.

In Oracle I create a directory object with this command:
CREATE OR REPLACE directory T_DIR AS 'T:';

Then I granted read and write privileges to my user
GRANT READ, WRITE ON DIRECTORY T_DIR TO <user>; (I tried also to set PUBLIC and SYSTEM)

But when I try to open a file with UTL_FILE with this command

file_handler := UTL_FILE.fopen( 'T_DIR', 'Dati.ini', 'r' ) ;

the error is
Exception: ORA-29283: operazione file non valida
ORA-06512: a "SYS.UTL_FILE", line 475
ORA-29283: operazione file non valida

If I change my directory object in 'C: est' (local folder) it works correctly.I tried also restart DB while shared folders were connected but with the same result.I tried, from sqlplus, the command host (dir t:) and it works (folder is accessible)

Is it an Oracle bug? Is it a Windows bug?