Let's Encryptのエラーの意味を教えてください

###前提・実現したいこと
一部のガラケーからサイトにアクセスできなくなりました。
SHA-2の証明書の影響かと思ったのですが、SHA-2切り替え後もしばらくはアクセスが可能だったようで
アクセスができなくなった直前と思われるLet's encryptのログを見たところ
何件かエラーがあったので、これのせいかな？とも思いつつ、結局わからずじまいです。
原因、解決方法を教えていただけますでしょうか？

###発生している問題・エラーメッセージ

2017-12-31 20:00:35,349:WARNING:certbot.renewal:Attempting to renew cert (.com) from /etc/letsencrypt/renewal/.com.conf produced an unexpected error: urn:acme:error:malformed :: The request message was malformed :: Unable to update challenge :: cannot update a finalized authorization. Skipping.
2017-12-31 20:00:35,352:DEBUG:certbot.renewal:Traceback was:
Traceback (most recent call last):
File "/opt/eff.org/certbot/venv/lib/python2.7/site-packages/certbot/renewal.py", line 425, in handle_renewal_request
main.renew_cert(lineage_config, plugins, renewal_candidate)
File "/opt/eff.org/certbot/venv/lib/python2.7/site-packages/certbot/main.py", line 743, in renew_cert
_get_and_save_cert(le_client, config, lineage=lineage)
File "/opt/eff.org/certbot/venv/lib/python2.7/site-packages/certbot/main.py", line 80, in _get_and_save_cert
renewal.renew_cert(config, domains, le_client, lineage)
File "/opt/eff.org/certbot/venv/lib/python2.7/site-packages/certbot/renewal.py", line 297, in renew_cert
new_certr, new_chain, new_key, _ = le_client.obtain_certificate(domains)
File "/opt/eff.org/certbot/venv/lib/python2.7/site-packages/certbot/client.py", line 318, in obtain_certificate
self.config.allow_subset_of_names)
File "/opt/eff.org/certbot/venv/lib/python2.7/site-packages/certbot/auth_handler.py", line 81, in get_authorizations
self._respond(resp, best_effort)
File "/opt/eff.org/certbot/venv/lib/python2.7/site-packages/certbot/auth_handler.py", line 134, in _respond
resp, chall_update)
File "/opt/eff.org/certbot/venv/lib/python2.7/site-packages/certbot/auth_handler.py", line 158, in _send_responses
self.acme.answer_challenge(achall.challb, resp)
File "/opt/eff.org/certbot/venv/lib/python2.7/site-packages/acme/client.py", line 230, in answer_challenge
response = self.net.post(challb.uri, response)
File "/opt/eff.org/certbot/venv/lib/python2.7/site-packages/acme/client.py", line 709, in post
return self._post_once(*args, **kwargs)
File "/opt/eff.org/certbot/venv/lib/python2.7/site-packages/acme/client.py", line 722, in _post_once
return self._check_response(response, content_type=content_type)
File "/opt/eff.org/certbot/venv/lib/python2.7/site-packages/acme/client.py", line 583, in _check_response
raise messages.Error.from_json(jobj)
Error: urn:acme:error:malformed :: The request message was malformed :: Unable to update challenge :: cannot update a finali
zed authorization

2017-12-31 20:00:35,353:ERROR:certbot.renewal:All renewal attempts failed. The following certs could not be renewed:
2017-12-31 20:00:35,353:ERROR:certbot.renewal: /etc/letsencrypt/live/******.com/fullchain.pem (failure)
2017-12-31 20:00:35,353:DEBUG:certbot.log:Exiting abnormally:
Traceback (most recent call last):
File "/opt/eff.org/certbot/venv/bin/letsencrypt", line 11, in <module>
sys.exit(main())
File "/opt/eff.org/certbot/venv/lib/python2.7/site-packages/certbot/main.py", line 861, in main
return config.func(config, plugins)
File "/opt/eff.org/certbot/venv/lib/python2.7/site-packages/certbot/main.py", line 797, in renew
renewal.handle_renewal_request(config)
File "/opt/eff.org/certbot/venv/lib/python2.7/site-packages/certbot/renewal.py", line 443, in handle_renewal_request
len(renew_failures), len(parse_failures)))
Error: 1 renew failure(s), 0 parse failure(s)