SQL & PL/SQL

On Oracle 11.2.0.1, I have a partitioned table with some partitions that need to be 'archived' (in terms of moving partitions to an 'archive' table).

I have a source table like:

CREATE TABLE IS_PODACI245
(
  ID_OBJEKTA_IDENTIFIKACIJA  NUMBER(10),
  ID_OBJEKTA                 NUMBER(20),
  DATUM                      TIMESTAMP(6)       NOT NULL,
  TZ                         NUMBER(3),
  DATA1                      NUMBER(10),
  DATA2                      NUMBER(6),
  DATA3                      NUMBER(10),
  DATA4                      NUMBER,
  DATA5                      T_NTCIP_CLIMATE_TABLE
)
NESTED TABLE DATA5 STORE AS IS_PODACI245_STORE_TABLE
TABLESPACE DATA
PARTITION BY RANGE (DATUM)
(  
  PARTITION P_201107 VALUES LESS THAN (TIMESTAMP' 2011-08-01 00:00:00')
    LOGGING
    NOCOMPRESS 
    TABLESPACE DATA,  
...
  PARTITION P_MAXVALUE VALUES LESS THAN (MAXVALUE)
    LOGGING
    NOCOMPRESS 
    TABLESPACE DATA
)
NOCOMPRESS 
NOCACHE
NOPARALLEL
MONITORING;


CREATE INDEX IDX_IS_PODACI245_KOMPLEKS ON IS_PODACI245
(ID_OBJEKTA_IDENTIFIKACIJA, ID_OBJEKTA, DATUM)
  TABLESPACE DATA
LOCAL (  
  PARTITION P_201107
    LOGGING
    NOCOMPRESS 
    TABLESPACE DATA,  
....
  PARTITION P_MAXVALUE
    LOGGING
    NOCOMPRESS 
    TABLESPACE DATA
)
NOPARALLEL;

CREATE OR REPLACE TYPE t_ntcip_climate_table as table of t_ntcip_climate_fmt;

CREATE OR REPLACE TYPE t_ntcip_climate_FMT as object
(  dev_index number(6)
,   dev_description varchar2(512)
,   dev_type number(10)
,   dev_status number(10)
,   dev_mfr_status varchar2(512)
,   dev_active number(3)
,   dev_test_activation number(10)
);
/

I would like to make exchange partition using stage table, and everything is going fine on all tables, but only on a few of them (listed source is one of them, and they're only tables with nested tables wihin), where I get an error.. but sometimes ;)

on a statement like:

ALTER TABLE IS_PODACI245_ARH EXCHANGE PARTITION P_201106  WITH TABLE IS_PODACI245_STAGE EXCLUDING INDEXES  WITHOUT VALIDATION;

I got an error:
ORA-00001: unique constraint (TXV.SYS_C0032911) violated

it's an unique index between parent and nested table.

what could cause that problem?