SSL_CLIENT_AUTH_SIGNATURE_FAILED

The client could not sign the CertificateVerify data of an SSL client auth handshake with the client certificate's private key.

Possible causes for this include the user implicitly or explicitly denying access to the private key, the private key may not be valid for signing, the key may be relying on a cached handle which is no longer valid, or the CSP does not allow arbitrary data to be signed.