Some issue with external table over preprocessor

I built an external table based on the output from a preprocessor. Got the following explanation, why it does not work:

      Trim whitespace same as SQL Loader
KUP-04027: file name check failed: /u01/app/oracle/nx/datamasking/nx_masking.dat
KUP-04017: OS message: Error 0
KUP-04118: operation "lstat64", location "skudmicdl"

All seemed to be ok. The script returned the output when called manually. Returned value was 0.
So what was wrong? The file I called was a link to the real script in fact. Once I dropped the link and copied the script source to the file with name referred within database, the problem vanished.

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';

Fractured block found during backing up datafile

{hostname}, Hex dump of (file 75, block 87980) in trace file /oracle/diag/rdbms/db1/db1/trace/db1_ora_23614.trc
Corrupt block relative dba: 0x12c157ac (file 75, block 87980)
Fractured block found during backing up datafile
Data in bad block:
 type: 6 format: 2 rdba: 0x12c157ac
 last change scn: 0x02d7.7dc0c4bc seq: 0x2 flg: 0x04
 spare1: 0x0 spare2: 0x0 spare3: 0x0
 consistency value in tail: 0xb1a20602
 check value in block header: 0xc892
 computed block checksum: 0x67fa
Reread of blocknum=87980, file=+DATA/db1/datafile/file1.343.956407117. found valid data : 

dbv file=+DATA/db1/datafile/file1.343.956407117 blocksize=8192 start=87980 end=87981 userid=sys/blbla

DBVERIFY: Release 11.2.0.4.0 - Production on Thu Mar 22 10:04:33 2018

Copyright (c) 1982, 2011, Oracle and/or its affiliates.  All rights reserved.

DBVERIFY - Verification starting : FILE = +DATA/db1/datafile/file1.343.956407117


DBVERIFY - Verification complete

Total Pages Examined         : 2
Total Pages Processed (Data) : 0
Total Pages Failing   (Data) : 0
Total Pages Processed (Index): 2
Total Pages Failing   (Index): 0
Total Pages Processed (Other): 0
Total Pages Processed (Seg)  : 0
Total Pages Failing   (Seg)  : 0
Total Pages Empty            : 0
Total Pages Marked Corrupt   : 0
Total Pages Influx           : 0
Total Pages Encrypted        : 0
Highest block SCN            : 0 (0.0)

Restricted session

-- at startup
startup restrict;

-- on running instance
alter system enable restricted session;

System altered.

-- to disable
alter system disable restricted session;

-- to check
select logins from v$instance; 

LOGINS
----------
RESTRICTED

ROLL_INVALID_MISMATCH (external link)

Great article on one of reasons to not share cursor https://magnusjohanssontuning.wordpress.com/2016/04/22/cursors-not-shared-due-to-roll_invalid_mismatch/

Password hash explained (external link)

An excellent article on password hashing in Oracle https://www.trustwave.com/Resources/SpiderLabs-Blog/Changes-in-Oracle-Database-12c-password-hashes/?page=1&year=0&month=0

Insightful article on connections db terminated by firewalls (external link)

Very insightful article on why/when firewalls throw connections https://li555li.blogspot.se/2014/09/.
In short - to prevent this one have to set SQLNET.EXPIRE_TIME to {timeout on firewall}/2-1 in minutes.