"Could not establish trust relationship for the SSL/TLS secure channel " That problem comes when you try to call https webservices in your web application. for solving this problem:

1. you need to install server certificate.
2. Alternate method is create static class which always validate to server certificate and returns true

you can do this by using this code:

using System.Net;
using System.Net.Security;
using System.Security.Cryptography.X509Certificates;


public static class WebConnect
{
/// <summary>
/// Sets the cert policy.
/// </summary>
public static void SetCertificatePolicy()
{
ServicePointManager.ServerCertificateValidationCallback
+= RemoteCertificateValidate;
}

/// <summary>
/// Remotes the certificate validate.
/// </summary>
private static bool RemoteCertificateValidate(
object sender, X509Certificate cert,
X509Chain chain, SslPolicyErrors error)
{
// trust any certificate!!!
return true;
}
}

before making connection to service, use SetCertificatePolicy method and your problem resolved.