Lets Encrypts Root Certificate is expiring – Incompatible clients!

Let’s Encrypt DST Root CA X3 expiration on Sept. 30 2021
Due to significant changes of the certificate provider Let’sEncrypt problems with some outdated clients can occur (web browser, email clients).
The Let’s Encrypt DST Root CA X3 will expire on September 30, 2021. That means those older devices that don’t trust ISRG Root X1 will start getting certificate warnings when visiting sites that use Let’s Encrypt certificates.
From this point on, clients which still use the old Let’s Encrypt root certificate can no longer verify server certificates and can no longer establish a secured TLS connection.

Details:
https://letsencrypt.org/docs/dst-root-ca-x3-expiration-september-2021/

Incompatible clients
Let’s Encrypt has introduced the new root certificate ISRG Root X1, but this is not stored in all clients. It is probably missing in older devices that have not received a firmware or software update for a long time.
Details:
https://letsencrypt.org/docs/certificate-compatibility/