ORA-12012: error on auto execute of job 5717
ORA-12537: TNS:connection closed~
ORA-06512: at "SYS.DBMS_SNAPSHOT", line 2254
ORA-06512: at "SYS.DBMS_SNAPSHOT", line 2460
ORA-06512: at "SYS.DBMS_SNAPSHOT", line 2429
ORA-06512: at line 1

SQL> select SCHEMA_USER,WHAT from dba_jobs where job=5717 AND BROKEN='N';

SCHEMA_USER WHAT
-------------------- ----------------------------------------
ACNTV dbms_mview.refresh('CREDIT_CARD','F');

select OWNER, OBJECT_NAME ,OBJECT_TYPE from dba_objects WHERE OBJECT_NAME='CREDIT_CARD';

OWNER OBJECT_NAME OBJECT_TYPE
-------------------- ------------------------------ -------------------
WNOUR CREDIT_CARD TABLE
ACNTV CREDIT_CARD TABLE
ACNTV CREDIT_CARD MATERIALIZED VIEW

SQL> set long 32767
SQL> SELECT MVIEW_NAME, QUERY FROM dba_MVIEWS WHERE MVIEW_NAME='CREDIT_CARD';

MVIEW_NAME
------------------------------
QUERY
--------------------------------------------------------------------------------
CREDIT_CARD
SELECT "NOLOGGING"."MOP_ID" "MOP_ID","NOLOGGING"."CARD_TYPE" "CARD_TYPE","NOLOGG
ING"."CARD_NUMBER" "CARD_NUMBER","NOLOGGING"."LAST4_DISPLAY" "LAST4_DISPLAY",
"NOLOGGING"."EXP_DATE" "EXP_DATE","NOLOGGING"."CC_PHONE" "CC_PHONE",
"NOLOGGING"."IS_DEFAULT" "IS_DEFAULT"
FROM "MOP"."CREDIT_CARD"@XX.XXX.COM "NOLOGGING"

We found this job is auto refresh MATERIALIZED VIEW "CREDIT_CARD" depend CONNECT remote database XX.XXXX.COM
 ORA-12537: TNS:connection closed
SO I think it maybe network error caused of this problem. If this error occured only once, we maybe able to ignore it.
if this frequent occurrence, we need check the communications network between two database OR do further research !