Web Services SSL Error : The remote certificate is invalid according to the validation procedure

This error usually occurs if your CA client certificate is not installed in the Trusted CA Root in the computer where the web service call is being made.

Follow the procedures namely
Step 5. Install the Certificate Authority's Certificate on the Client Computer

as described in Patterns and Practices:
Building Secure ASP.NET Applications: Authentication, Authorization, and Secure Communication

Happy Programming,
Anton