This should not be related with your initial viewstate error. The purpose of the existingResponse attribute is to leave the errors sent by the application untouched. MonoX does not use custom error pages from IIS.
There could be many reasons for the original error, and I must admit that I don't remember this being reported, especially so early in the installation process. One of the better explanations can be found here: http://blogs.msdn.com/b/tom/archive/2008/03/14/validation-of-viewstate-mac-failed-error.aspx
However, I don't think it covers everything. Which version of MonoX are you using? Did you download it from this site? Can this server be accessed remotely so we could take a look?