また次のメールが届いた．

Let’s Encrypt certificate expiration notice for domain “itl.mydns.jp” (and 4 more)

Hello,
Your certificate (or certificates) for the names listed below will expire in 20 days (on 30 Aug 19 02:53 +0000). Please make sure to renew your certificate before then, or visitors to your website will encounter errors.
We recommend renewing certificates automatically when they have a third of their total lifetime left. For Let’s Encrypt’s current 90-day certificates, that means renewing 30 days before expiration. See https://letsencrypt.org/docs/integration-guide/ for details.
itl.mydns.jp
…
mish.myds.me
For any questions or support, please visit https://community.letsencrypt.org/. Unfortunately, we can’t provide support by email.
For details about when we send these emails, please visit https://letsencrypt.org/docs/expiration-emails/. In particular, note that this reminder email is still sent if you’ve obtained a slightly different certificate by adding or removing names. If you’ve replaced this certificate with a newer one that covers more or fewer names than the list above, you may be able to ignore this message.
If you are receiving this email in error, unsubscribe at http://mandrillapp.com/track/unsub.php?u=30850198&id=fab156addb7b4097869d6fbc1603e9d3.RsjpgIgXcXbPKK4%2Fg4vK9vppkUw%3D&r=https%3A%2F%2Fmandrillapp.com%2Funsub%3Fmd_email%3Diwao.n%2540ae.auone-net.jp
Regards,
The Let’s Encrypt Team

Synology NAS の DSM で更新してくれるはずなのにうまく動作しなかったのか？
DSM の「コントロールパネル」－「セキュリティ」－「証明書」から対象のものを選んで「右クリック」－「証明書を更新」．

2021/07 NAS の証明書の更新

Let’s Encrypt 証明書を手動で更新していて，
「最大の認証要請が、このドメイン名に到着しました。」のエラーに．

作成や更新を，何度も要求した影響か？

DS115j は最近自動で更新されているみたい．
DS116 は，2019/01 に更新した時のまま．
その後どうなったかを試していて，DS115j の証明書がうまく取得できていない．

2019/04/03　DS116 の方は更新されたみたい．

2020/04/14
その後更新されないこともあったが，今は特に問題なし．

Let’s Encrypt からのメール．
Hello,
Action may be required to prevent your Let’s Encrypt certificate renewals from breaking.
If you already received a similar e-mail, this one contains updated information.
Your Let’s Encrypt client used ACME TLS-SNI-01 domain validation to issue a certificate in the past 60 days. Below is a list of names and IP addresses validated (max of one per account):
mish.myds.me (27.92.169.109) on 2018-12-30
TLS-SNI-01 validation is reaching end-of-life. It will stop working temporarily on February 13th, 2019, and permanently on March 13th, 2019.
Any certificates issued before then will continue to work for 90 days after their issuance date.
You need to update your ACME client to use an alternative validation method (HTTP-01, DNS-01 or TLS-ALPN-01) before this date or your certificate renewals will break and existing certificates will start to expire.
Our staging environment already has TLS-SNI-01 disabled, so if you’d like to test whether your system will work after February 13, you can run against staging: https://letsencrypt.org/docs/staging-environment/
If you’re a Certbot user, you can find more information here:
https://community.letsencrypt.org/t/how-to-stop-using-tls-sni-01-with-certbot/83210
Our forum has many threads on this topic. Please search to see if your question has been answered, then open a new thread if it has not:
https://community.letsencrypt.org/
For more information about the TLS-SNI-01 end-of-life please see our API
announcement:
https://community.letsencrypt.org/t/february-13-2019-end-of-life-for-all-tls-sni-01-validation-support/74209
Thank you,
Let’s Encrypt Staff

どうも TLS-SNI-01 が利用できなくなると言うことらしい．
それで Synology NAS と Let’s Encrypt で検索すると
Synology TLS-SNI-01 End of Life Email? – Help – Let’s Encrypt …
今（2019/01/27）の時点では，はっきりした対応方法が読み取れなかった．
取りあえず，「証明書の更新」を行って 3 ヶ月後に延ばした．

2019/04/03　自動的に更新されたことを確認．