A few people have seen this error after setting the ClientCertificate property and connecting using client authentication in FTPS, generally using the SSLFTPCertificate.CreateFromPFX method to create the ClientCertificate instance.
It seems that if the client certificate file has more than one certificate in it (e.g. an intermediate certificate), the private key in the file fails to get associated with the correct client certificate, and this error results.
The way to deal with this is to ensure the PFX file contains only the client certificate and its private key. This can be done via OpenSSL - email us for more details.