kkzifr3g: Encountered error ORA-12057.

The solution is as follows:

-- as we initially do not know what is happening exactly
alter system set events '12057 trace name errorstack level 3';
-- and wait until hit the issue again (note the command above dumps only 1st occurrence in a session)

[.. this is from alert.log - this time much more info ..]
Mon Mar 26 10:16:01 2018
kkzifr3g: Encountered error ORA-12057.
Mon Mar 26 10:16:01 2018
Dumping diagnostic data in directory=[cdmp_20180326101601], requested by (instance=1, osid=60555319 (J002)), summary=[abnormal process termination].
Mon Mar 26 10:16:02 2018
Errors in file /u01/app/oracle/diag/rdbms/db1/db1/trace/db1_j005_24117326.trc:
ORA-12057: materialized view or zonemap "TEST"."MV_DATA1" is invalid and must use complete refresh
[..]

-- and even more data in trace, but we care here mostly about the mv name
-- then ORA-12057 During Fast Refresh of Aggregate Materialized View (Doc ID 1399651.1)

-- to clean up
alter system set events '12057 trace name context off';