The certificate for {Subdomain.Name} has expired. Expired certificates can not be validated and can not guarantee ownership of a subdomain. This means a user could not distinguish access to the correct website and a fake one and opens users of this website to the risk of an MITM attack
Recommended Mitigation
Access the domain through a modern browser to see if the user is alerted on the certificate's status. Review the certificate and renew it if it is indeed expired.