Prueba así y comenta tus resultados.
SELECT T0.[DocNum], T0.[DocDate], T0.[DocTotal],T1.USER_CODE
FROM OQUT T0 INNER JOIN OUSR T1 ON T0.UserSign = T1.INTERNAL_K
WHERE T1.USER_CODE = (Select top 1 UserCode from USR5 where SessionID=@@spid order by Date desc,Time desc )