Solution:

https://github.com/goharbor/harbor/issues/12003

postgresql.conf:
sudo sed 's/max_connections =.*/max_connections=999/g'

This issue is because the postgres database connection exceeded.
The default max connection is 100.
However ever it's still not working even set database.max_open_conns in harbor.yaml.
You have to also manually edit postgresql.conf like
sudo sed 's/max_connections =.*/max_connections=999/g' /data/database/postgresql.conf
and restart harbor-db

track db issue with #12124