I had the same problem with Unity (which also uses mono) and this post helped me to solve it.
Just add the following line before making your request:
ServicePointManager.ServerCertificateValidationCallback = MyRemoteCertificateValidationCallback;
And this method:
public bool MyRemoteCertificateValidationCallback(System.Object sender,
X509Certificate certificate, X509Chain chain, SslPolicyErrors sslPolicyErrors)
{
bool isOk = true;
// If there are errors in the certificate chain,
// look at each error to determine the cause.
if (sslPolicyErrors != SslPolicyErrors.None) {
for (int i=0; i<chain.ChainStatus.Length; i++) {
if (chain.ChainStatus[i].Status == X509ChainStatusFlags.RevocationStatusUnknown) {
continue;
}
chain.ChainPolicy.RevocationFlag = X509RevocationFlag.EntireChain;
chain.ChainPolicy.RevocationMode = X509RevocationMode.Online;
chain.ChainPolicy.UrlRetrievalTimeout = new TimeSpan (0, 1, 0);
chain.ChainPolicy.VerificationFlags = X509VerificationFlags.AllFlags;
bool chainIsValid = chain.Build ((X509Certificate2)certificate);
if (!chainIsValid) {
isOk = false;
break;
}
}
}
return isOk;
}