This temporary intermediate certificate was used in years past as part of a compatibility chain for older devices. The expired certificate in question is the “DigiCert High Assurance EV Root CA' certificate.

Server-to-server connections on Windows environments, where one server still has the legacy certificate installed.Clients (mainly OS X) with the expired intermediate installed in their local keychain.The problem can affect any client platform with a locally cached or installed intermediate certificate. The problem is related to a locally installed legacy intermediate certificate that is no longer used and no longer required for the certificate installation. On Jat 12:15 PM, some customers and users on sites secured by DigiCert reported that they were getting an untrusted certificate error. For more information on root certificates, read The Impacts of Root Certificate Expiration.

Note that intermediate certificates rely on root certificates. June 2020 Update: With a large number of sites affected by the recent expiring of a root certificate, we thought it would be valuable to again share this guide on intermediate TLS/SSL certificates in the certificate chain.