Call to activate session gives error when using X509IdentityToken|OPC UA Implementation: Stacks, Tools, and Samples|Forum|OPC Foundation

We are using OPC java stack (opc-ua-stack-1.4.1.jar)  in our client.

We are trying to create UserIdentityToken for the UserTokenType Certificate. Below is the code snippet. 

SignatureData signatureData = new SignatureData();
X509IdentityToken x509IdentityToken = EndpointUtil.createX509IdentityToken(endpoint, session.getServerNonce(), session.getClientCertificate(), session.getClientPrivateKey().getPrivateKey(), signatureData);
sessionChannel.activate(x509IdentityToken, signatureData); 

The activate method returns an error 'The session id is not valid'. What is it that we are doing wrong? Just a note - the same client certificate is used to create SecureChannel and Session with MessageSecurityMode as SignAndEncrypt.