The original error message didn't make sense to me until later. It said "Validation of viewstate MAC failed. If the application is hosted by a web farm of cluster, ensure that
At first I thought something was wrong with machine.config or web.config. I searched for the error message and read this forum post about adding changing web.config to add entries to the pages tag:
That didn't work, so I found the request where the errors happened and tried deleting the Form Post Parameters that referred to __VIEWSTATE=
That didn't work, so I read about __VIEWSTATE this article explained that Viewstate contains user entered information encrypted in a hidden POST form field so users don't have to re-enter information after moving to another page.
Then I recorded a web test to get the updated encrypted VIEWSTATE=/whGSTdfdbdjdfy and copied the updated value to the other web tests. I also found a post that tells how to make the viewstate dynamic, but I only have Visual Studio installed on one machine now.