I have a simple web service call, generated by a .NET (C#) 2.0 windows app, via the web service proxy generated by Visual Studio, for a web service also written in C# (2.0). This has worked for several years, and continues to do so at the dozen or so places where it is running.
A new installation at a new site is running into a problem. When attempting to invoke the web service, it fails with the message saying:
Could not establish a trust relationship for the SSL/TLS secure
Thoughts (based on pain in the past):