Forms :: FRM-40733 - PL/SQL Built-in DBMS_ERROR_CODE Failed

I have a client who is facing the above error. Everything used to work perfectly but since a week, he gets this error message on a specific FORM. No updates have been made to the FORM.

The FMXs are stored on the database server(10G R3, Windows 2003 R2) and the Client terminals access them through a network drive.When executing the conflicting FORM on the server, no error.

I call the DBMS_ERROR_CODE routine from a FORM-LEVEL ON-ERROR trigger :

DECLARE
ERR NUMBER;
BEGIN
IF DBMS_ERROR_CODE=-1 THEN
MSG('Client existant',ERR,'E');
ELSE

[code]....

Code for procedure MSG :

PROCEDURE MSG(TXT_MSG IN VARCHAR2 , RET_REP OUT NUMBER , TYPE_MSG IN CHAR DEFAULT 'I' ,
LABEL_BUT1 IN VARCHAR2 DEFAULT 'OK' , LABEL_BUT2 IN VARCHAR2 DEFAULT NULL ,
LABEL_BUT3 IN VARCHAR2 DEFAULT NULL) IS
VL_ALERT ALERT;
BEGIN

[code]....