The displayed error may be shown when your certificate setup is out of date - if you’re using Windows 7, you have to renew them manually (since Microsoft doesn’t support Windows 7 anymore). A lot of websites rely on those certificates for authentication purposes. Please check whether the DST Root CA X3 certificate is still valid on your computer, and if you have the new ISRG Root X1 already installed by any chance. You can do so by running certmgr.msc in the command line.
If you don’t have the ISRG Root X1 certificate set up yet, here’s a quick-how to:
How to setup the ISRG Root X1 certificate on Windows 7/10:
Download the .der ISRG Root X1 certificate file from https://letsencrypt.org/certificates/. If you can’t access the website, try downloading this file to a different machine, and transferring it (e.g. pendrive).
Click the downloaded file to start the installation process. (or left click and choose install)
Click install certificate.
Click “Next” on the next window
Select “Place all certificates in the following store”:
Click “Browse” and select “Trusted Root Certification Authorities”. Confirm the changes by pressing “OK” and “Next”:
Finish the certificate installation by pressing “Finish”:
You can press the Windows key + R to bring up the Run command, and type certmgr.msc and press Enter to open the certmgr.msc window and to check if the installation was successful (the ISRG Root X1 Certificate should appear under the Trusted Root Certification Authorities Certificate Store)
(You can download new DST Root CA X3 certificate replacement following the link in the same website.)