In a multitier PKI hierarchy, such as a two-tier or three tier PKI, the Enterprise PKI application will display "Unknown Error" for the CA certificate of a subordinate certification authority (CA) if the root CA or any parent CAs certificate has been revoked.See CA Certificate shows Unknown Error for a subordinate CA in Enterprise PKI (PKIView.msc) on the TechNet Wiki http://social.technet.microsoft.com/wiki/contents/articles/18102.ca-certificate-shows-unknown-error-for-a-subordinate-ca-in-enter
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