Could not create SSL/TLS secure channel inside my web service

[=?Utf-8?B?RmlsaXBwbyBCZXR0aW5hZ2xpbw==?=]

I have developed a web service under ASP.NET2. this web service call
another web service which need a SSL connection.

Therefore I pass, to be able to connect, I pass credential and certificate

X509Certificate x509cert =
new X509Certificate(@"C:\Visual Studio
Projects\DprGateway\Providers\Summit\Code\Src\Dpr. Gateway.Providers.Summit\Log\ais-services.p12", "password");

wsSummitProvider.ClientCertificates.Add(x509cert);
//Set network credential
wsSummitProvider.Credentials = new NetworkCredential("lfsuser", "lfsuser");

…..
Then I call my web service

All works fine under the VS2005 built-in server, but it doesn’t under IIS 5.

The error that I get is:

The request was aborted: Could not create SSL/TLS secure channel.

Now even under IIS I can read the certificate, I can see the public key! so
in not the access to the certificate the problem.

It is something else, it seems that IIS denies the creation of the SSL
channel. The problem is not on the clients of my web service but it is
connecting from my web service to web service!
Many thanks,
Filippo