ORA-01830: date format picture ends before converting entire input string

create table    tq84_tab (
   dat    date,
   xyz    varchar2(10)
);

insert into tq84_tab
select
   date '2020-01-01' + level,
   to_char(level, 'fmxxxxxx')
from
   dual connect by level <= 10000;

alter session set nls_date_format = 'dd.mm.yyyy';

select xyz from tq84_tab where dat = '17.08.2021';
--
-- XYZ       
-- ----------
-- 252

alter session set nls_date_format = 'yyyy-mm-dd';

select xyz from tq84_tab where dat = '17.08.2021';
--
-- ORA-01830: date format picture ends before converting entire input string
--