INSERT INTO STS_RESP_LOG(STS_REQ_LOG_SYSTEM_ID,HSTRY_FLG, SNGL_STS, FRQNCY, CRT_DATE )
                 VALUES (                    ?,        ?,        ?,      ?, sysdate);

I've never heard of such an instance where we insert a ZERO (or any other non-null value) into a column that is not specified in the SQL statement.

Things to look for:

- an erroneous triggers on the table (don't forget, triggers can be in a different schema to the table)
- erroneous DEFAULT clause

If all fails, you could explicity ADD a trigger to the table temporarily at AFTER ROW level, with something like:

if :new.msg_id is not null then
  raise_application_error(-20000,'WE FOUND ONE!');
end if;

and use things like UTL_CALL_STACK and the like to capture everything to see where that particular insert was different to the others.