Forms :: FRM-40654 On Some Rows Only?

I developed a small app using forms, which queries a table in our DB and allows the user to edit some of the values in a grid-like format. Sometimes I get this FRM-40654 error when trying to key in a value in a column. Sometimes means that it may happen or not happen at all, but also that it happens on some of the queries rows and not on others. IN one case I have 4 rows and it happens on all but the 2nd The table has a small trigger on it:

CREATE OR REPLACE TRIGGER MFI_CT_T_TRG1
before UPDATE ON MFI_CITY_TAX_T
REFERENCING OLD AS old NEW AS new
FOR EACH ROW
BEGIN
:new.modified_on := sysdate;
:new.modified_by := pms_p.logged_uid;
END;
/

But I tried already to remove it and it didn't work.I set the DML Returning Values of the block to Y, still the same.