Are you sure that the format is YYYYMMDD and not DDMMYYYY?
Could the format be reversed for some of the data?

Try
SELECT LENGTH(E1_VENCTO), count(*)
from SE1010
group by LENGTH(E1_VENCTO)
order by 1;

and

SELECT substr(E1_VENCTO, 1, 4), count(*)
from SE1010
group by substr(E1_VENCTO, 1, 4)
order by 1;

You might see some data that you did not expect.