On September 30, the DST Root CA X3 root certificate used by Let's Encrypt SSL certificates expired. As a result, older browsers and devices that do not support the new ISRG Root X1 root certificate may no longer trust Let's Encrypt certificates. Visitors using outdated devices may see security warnings.
Affected systems may include: Windows XP before SP3, macOS before 10.12, iOS before 10, Android before 2.3, Ubuntu before 16, Debian before 8, and older smart TVs or smart home devices.
Solutions: - Update the OS or device firmware to the latest version if possible. - Use Firefox version 50 or higher, which ships with its own certificate store and does not depend on the operating system. - Order a different SSL certificate.