Certbot installed and running with LetEncrypt, but Browsers show warnings

[DaveCoventry]

My domain is: coventrysoap.co.za cluett.co.za magico.co.za docndraft.co.za + others

I ran this command: certbot --nginx

It produced this output:

• Congratulations! Your certificate and chain have been saved at:
  /etc/letsencrypt/live/docndraft.co.za-0001/fullchain.pem
  Your key file has been saved at:
  /etc/letsencrypt/live/docndraft.co.za-0001/privkey.pem
  Your cert will expire on 2021-05-10. To obtain a new or tweaked
  version of this certificate in the future, simply run certbot again
  with the "certonly" option. To non-interactively renew all of
  your certificates, run "certbot renew"

My web server is (include version):nginx version: nginx/1.14.0 (Ubuntu)

The operating system my web server runs on is (include version):Ubuntu 18.04

I can login to a root shell on my machine (yes or no, or I don't know):yes

I'm using a control panel to manage my site (no, or provide the name and version of the control panel):no

The version of my client is (e.g. output of certbot --version or certbot-auto --version if you're using Certbot):certbot 0.31.0

Error on Firefox:

Secure Connection Failed

An error occurred during a connection to cluett.co.za. You are attempting to import a cert with the same issuer/serial as an existing cert, but that is not the same cert.

Error code: SEC_ERROR_REUSED_ISSUER_AND_SERIAL

The page you are trying to view cannot be shown because the authenticity of the received data could not be verified.
Please contact the website owners to inform them of this problem.

https://www.ssllabs.com/ssltest/analyze.html?d=cluett.co.za

[_az]

@DaveCoventry

Error code: SEC_ERROR_REUSED_ISSUER_AND_SERIAL

It doesn't seem likely that Let's Encrypt have reused a serial number on a certificate. At least, I can't find any for that domain.

Are you running any antivirus software on the computer where this error is happening?

There are some reports that antivirus proxies, which intercept and replace SSL certificates in order to scan HTTPS traffic, can cause this error to happen.

Your site loads without errors for me in Firefox.

[DaveCoventry]

Thanks.

I've moved the sites to another server as they are production sites and they were already offline for an uncomfortable period of time (or, if not actually offline, then warning off users which amounts to the same thing).

However, your post was well before that, though, so I presume that these errors are only raised from my machine which is running Ubuntu 20.10. As you can imagine, I'm not using anti-virus software.

I am, however, accessing the Internet from a fairly rural area in South Africa and there have been issues with my SSH sessions online, so it is a distinct possibility that my flaky internet connection is to blame.

Thanks once again.