Could not establish secure channel for SSL/TLS

[Scott McFadden]

I have a .NET client application that frequently connects to a ASP.Net
Webservice to transfer small files over HTTPS/SOAP. The client app just
uses the standard proxy that .NET generates based on SoapHttpClientProtocol.
Sometimes, the transfer works just fine and but many times, the client
application gets the following error when trying to communicate with the web
service:

Exception Message: The underlying connection was closed: Could not establish
secure channel for SSL/TLS.

I am not quite sure where to begin troubleshooting this problem. Has anyone
else had problems with SSL/Soap using .NET?

thanks