This can be caused by overlapping legal status records. There is a change in Mosaic 21.1 that should prevent overlapping records from being added from the Mosaic front end.
If you do see this error in the populate log, the query below can be used to identify records with overlapping dates.
select
pls.person_id, pls.id legal_status_id, pls_.id other_ls_id,
pls.start_date, pls.end_date,
pls_.start_date, pls_.end_date,
pls.legal_status, pls.legal_status other_legal_status
from
person_legal_statuses pls
inner join person_legal_statuses pls_
on pls_.person_id = cast(pls.person_id as integer)
and pls_.start_date < coalesce(pls.end_date, pls_.start_date + 1)
and pls.start_date < coalesce(pls_.end_date, pls.start_date + 1)
and pls_.id > pls.id
where
( pls.end_date is null
or
pls.start_date < pls.end_date + 0.99999)
and
( pls_.end_date is null
or
pls_.start_date < pls.end_date + 0.99999);