The revocation status of a certificate of the chain is not known: an OCSP responder answered unknown, or, when RequireRevocationStatus is set, no CRL or OCSP response that applies at the validation time was found (ISO 32000-2 §12.8.3.4.6 to §12.8.3.4.8; RFC 6960 §2.2).
RevocationUnknown = 24