Trigger problem (confusing)

Hoj!

I have the next trigger:

---
CREATE OR REPLACE TRIGGER DPS.POST_PREDOBRM_TBI_01
  BEFORE INSERT ON DPS.POST_PREDOBRM
  FOR EACH ROW

DECLARE
  -- Variables
  LS_LOK_ID		DPS.LOKACIJA.LOK_ID%TYPE;
  LN_DOK_ID		DPS.DOKUMENT.DOK_ID%TYPE;

BEGIN
  SELECT NVL(T1.DOK_LOK_ID, ''), NVL(T1.DOK_ID, 0)
    INTO LS_LOK_ID, LN_DOK_ID
    FROM DP.DOK_FEP T1,
         DPS.DOKUMENT T2
    WHERE (T1.STEV_PREDOB = :NEW.STEV_PREDOB)
          AND (T2.LOK_ID = T1.DOK_LOK_ID)
          AND (T2.DOK_ID = T1.DOK_ID)
          AND (T2.DST_ID = 'FO ');

  INSERT INTO DPS.PRS_POG_SRED
    (DOK_LOK_ID, DOK_ID,
     LET_ID, POS_ID, STEV_PREDOB,
     PGR_ZNE_UPO, PGR_ZNE_IZPL630,
     PGR_ZNE_IZPL637, PGR_ZNE_SPEC, PGR_ZNE_ODR)
    VALUES (LS_LOK_ID, LN_DOK_ID,


            :NEW.LET_ID, :NEW.POS_ID, :NEW.STEV_PREDOB,

            :NEW.ZNES_PREDOB, :NEW.PGR_ZNE_IZPL630,

            :NEW.PGR_ZNE_IZPL637, :NEW.PGR_ZNE_SPEC,

            :NEW.PGR_ZNE_ODR);


  RETURN;

  -- Napaka!
  EXCEPTION
    WHEN NO_DATA_FOUND THEN
      RAISE_APPLICATION_ERROR(-20000, 'Pri INSERT v ...');
    WHEN INVALID_NUMBER THEN
      RAISE_APPLICATION_ERROR(-20001, 'Pri INSERT v ...');
    WHEN OTHERS THEN
      RAISE_APPLICATION_ERROR(-20002, 'Pri INSERT v POST_PREDOBRM: neznana
napaka!');
END;
/
---

The problem is, that when I insert into table "POST_PREDOBRM" I ALWAYS get
the ORA-20002 Error.
Why?
I tried the first SELECT statment manualy, and all worked OK. Whatever I
do, I still get the ORA-20002 exception, which is in my example the
OTHERS!!!
Any ideas?

Thank you.

-- 
Boštjan Štupica