If you are using outdated devices or outdated software,

you can observe an error on your site that it’s not secure,

even though the certificate was installed successfully.

This is caused by an expired DST Root CA X3 root certificate

On September 30, the DST Root CA X3 certificate expired,

that verified the security of Let’s Encrypt certificates

A new certificate that validates certificates from Let’s Encrypt

is codenamed ISRG Root X1.

Accordingly, all devices and operating systems that have been released

before the ISRG Root X1 certificate was generated

do not consider Let’s Encrypt certificates to be secure and may generate an error.

Legacy systems that may have access problems:

– Windows XP up to SP3 version

– macOS up to version 10.12

– iOS to version 10

– Android up to version 2.3

– Ubuntu up to version 16

– Debian up to version 8

– Older models of smart TVs and IOT devices.

To fix this situation you can install ISRG Root X1 certificate on your system yourself.

Download it from this link – https://letsencrypt.org/certificates

Select the .der certificate there

Active ISRG Root X1 (RSA 4096, O = Internet Security Research Group, CN = ISRG Root X1)Self-signed: der, pem, txt

When you run this certificate, a certificate installation dialog box should open:

Why did sites with free Let's Encrypt SSL certificates stop working?

 SSL Certificates
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