Error "HTTP/1.1 401 Unauthorized" with basic authentication in android - EWS 2010

I refer to this link to request to the server. The problem is sometime (not always, about 20% - 30%, means sometime I can get successful response), I got the 401 error and server response basic authorization challenge expected, but not found.

Here is my code:

 HttpRequestInterceptor preemptiveAuth = new HttpRequestInterceptor()
{
    public void process(final HttpRequest request, final HttpContext context)
            throws HttpException, IOException
    {
        AuthState authState = (AuthState) context
                .getAttribute(ClientContext.TARGET_AUTH_STATE);
        CredentialsProvider credsProvider = (CredentialsProvider) context
                .getAttribute(ClientContext.CREDS_PROVIDER);
        HttpHost targetHost = (HttpHost) context
                .getAttribute(ExecutionContext.HTTP_TARGET_HOST);

        if (authState.getAuthScheme() == null)
        {
            AuthScope authScope = new AuthScope(targetHost.getHostName(),
                    targetHost.getPort());
            Credentials creds = credsProvider.getCredentials(authScope);
            if (creds != null)
            {
                authState.setAuthScheme(new BasicScheme());
                authState.setCredentials(creds);
            }
        }
    }
};

Here is my authentication:

    HttpPost httpPost = new HttpPost(SERVER_AUTH_URL);
        httpPost.setHeader("Content-type", "text/xml; charset=utf-8");
    httpPost.setHeader("Keep-Alive", "300");
    httpPost.setHeader("Connection", "Keep-Alive");     
    StringEntity se = new StringEntity(myRequest, "UTF-8");
    httpPost.setEntity(se);
    se.setContentType("text/xml");
    se.setContentEncoding("gzip,deflate");

        //add preemptiveAuth
        client.addRequestInterceptor(preemptiveAuth, 0);
        //Set the proxy
    ProxySelectorRoutePla