I have noticed that the CODE field size is 15,
 TMPCODE is 15 and TMPSOCIETE is 5.

 CODE=TMPCODE+TMPSOCIETE

With the above line, you are assigned a varchar(20) to a varchar(15), aren´t you?
Is this right?
 

 CREATE PROCEDURE NEW_CODE_HISTO
RETURNS (CODE VARCHAR(15))
AS
DECLARE VARIABLE TMPCODE VARCHAR(15);
DECLARE VARIABLE TMPSOCIETE VARCHAR(5);
...
...
...  CODE=TMPCODE+TMPSOCIETE;

Cheers,
Clément