Validation of viewstate MAC failed [duplicate]

Answer 1

I asked a similar question a while ago. Did you see this post? Another thing that helped me with this error was setting a machineKey. See this Microsoft Page about the problem and setting a Machine Key.

Answer 2

I started having this problem today.

Long story short - I'd copied some of the form code from a similar page and forgotten to change the form action:

<form id="Form1" action="search.aspx" method="post" runat="server">

</form>

The form on results.aspx should have been:

<form id="Form1" action="results.aspx" method="post" runat="server">

</form>

I would recommend anyone else check this and also any inherits values before delving into config and other settings.