A Let's Encrypt root certificate has expired yesterday. This can lead to various errors TLS error for old clients, the most common being "Certificate date is invalid".
Our Let's Encrypt certificates already provide the up-to-date Let's Encrypt chain but some older clients might not be able to trust that new chain because they don't have the new root Certificate Authority in their trustore. If you are in this situation with clients you can't update, we can sell certificates that will be trusted by those older clients. You can contact us on the support with the domains you need to protect.
You can also find more information about this expiration on Let's Encrypt website: https://letsencrypt.org/docs/dst-root-ca-x3-expiration-september-2021/