What is this?

  x_receipt_link_URL.Value =  SilentForm.aspx;

Need to be "Post", "Link" will never work.

x_receipt_link_method.Value ="Link";

So even if you have EnableViewStateMAC="false" on both those pages. It it still getting

Validation of viewstate MAC failed. If this application is hosted by a Web Farm or cluster, ensure that <machineKey> configuration specifies the same validationKey and validation algorithm. AutoGenerate cannot be used in a cluster

When you click on "click here to return to our home page"