The Viewstate is the most common beastie that happens when trying to load test an ASP .Net application. Just search this forum to see the various and vicious ways that this happens. More than likely your fix is not helping because viewstate is dynamic and hard-coding it will only lead to different errors.
There seems to be a basic problem with viewstate in your load test. The message about the viewstate MAC, which relates to encryption, is probably a red herring. The real problem is that the viewstate you're sending back to the server is just plain wrong.
The most common cause of this relates to websites that use frames or iframes. This causes viewstate to be sent back to the server for the wrong page, because ANTS Load only has one slot for viewstate data, it is not maintained per-page. Therefore, if your web browser session recorded a request for page a, then b, then c, and your form posts to page b, the viewstate for page c gets posted back to page b.
So what you would need to do is edit your script so that the GET request for page b happens right before the POST to page b. You can try the ANTS Script tester from http://labs.red-gate.com
to try to figure out exactly where the breakdown is occurring.