How do I debug the sudden emergence of dns-linode "Authentication error" on certbot renewal?

A certbot command that has worked for over a year has suddenly returned "Authentication error". I've made no changes to the:

• DNS on linode
• --dns-linode-credentials
• list of domains in the certbot command.

Moreover, /var/log/letsencrypt/letsencrypt.log, at DEBUG level shows the plugin is located:

Entry point: dns-linode = certbot_dns_linode.dns_linode:Authenticator
Initialized: <certbot_dns_linode.dns_linode.Authenticator object at 0x7f7d5fc91c50>
Prep: True
2020-06-01 17:39:13,304:DEBUG:certbot.plugins.selection:Selected authenticator <certbot_dns_linode.dns_linode.Authenticator object at 0x7f7d5fc91c50> and installer None
2020-06-01 17:39:13,304:INFO:certbot.plugins.selection:Plugins selected: Authenticator dns-linode, Installer None
2020-06-01 17:39:13,308:DEBUG:certbot.main:Picked account: <Account(RegistrationResource(body=Registration(key=None, contact=(), agreement=None, status=None, terms_of_service_agreed=None, only_return_existing=None, external_account_binding=None), uri='https://acme-staging-v02.api.letsencrypt.org/acme/acct/11763618', new_authzr_uri=None, terms_of_service=None), 1e7db8a55dc8b8305dd70a871befbf3d, Meta(creation_dt=datetime.datetime(2019, 12, 10, 19, 56, 17, tzinfo=<UTC>), creation_host='li1186-128.members.linode.com'))>

This is rather an emergency as I just discovered the certificate has expired.