It would be useful to know the exact error condition being generated. However I would work towards simplifying the expression by
a) including an additional OLE SRCE dataset which included the basic transformations, ie
SELECT 1 AS CASE_STATUS
,'A1021002' AS RESULT
UNION ALL
SELECT 2 AS CASE_STATUS
,'A1021002' AS RESULT
etc etc
UNION ALL
SELECT 10 AS CASE_STATUS
,null AS RESULT
etc etc
b) doing a merge join wth the primary data source to return the results for everything other than case_status 10
c) having the derived column expression to include the additonal logic for Case_Status 10 and the associated Reject_Reason (s)