MOS note 204127.1 has a big list of things which may lead to ORA-12034 errors.
If you've narrowed it down to something purging the MV log, be aware that there are several DDL commands that can purge the log. Notably truncate table and partition operations:
create table t (
c1 int primary key , c2 int
) partition by range ( c1 ) interval ( 1 ) (
partition p1 values less than ( 2 )
);
create materialized view log on t
with rowid, primary key ( c2 )
including new values;
insert into t
with rws as (
select level x from dual
connect by level <= 10
)
select x, x from rws;
commit;
select count(*) from mlog$_t;
COUNT(*)
10
alter table t
truncate partition p1;
select count(*) from mlog$_t;
COUNT(*)
0
So check to see if you've got any maintenance operations like these on the table. Failing this, look to see if you can add some auditing to the MV log to capture what exactly purges it.