Monitoring my global exception logs this error seems to be impossible to remove no matter what I do, I thought I finally got rid of it but it's back again. You can see a strack trace of the error on a similar post here.

Notes about the environment:

IIS 6.0, .NET 3.5 SP1 single server ASP.NET application

Steps already taken:

  <system.web>
    <machineKey validationKey="big encryption key"
      decryptionKey="big decryption key"
      validation="SHA1" decryption="AES" />

In my Page Base for all of my pages

  protected override void OnInit(EventArgs e)
  {
    const string viewStateKey = "big key value";

    Page.ViewStateUserKey = viewStateKey;
  }

Also in the source of the page I can see that all of the ASP.NET generated hidden fields are correctly at the top of the page.