Invalid State Problem

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VB

ASP.NET

I am making modifications of a current ASP.NET site. I'm somewhat new to ASP.NET so I am only doing little things. I only added a button and a function:

<form>
  <asp:Button Text="OPEN TEST" runat="SERVER" OnClick="TEST_COMMAND" ID="BUTTEST" />
</form>

I made the TEST_COMMAND but it suppose to change the button's text.
When I click the button I get the following error:

Validation of viewstate MAC failed. If this application is hosted by a Web Farm or cluster, ensure that <machineKey> configuration specifies the same validationKey and validation algorithm. AutoGenerate cannot be used in a cluster.

I've been reading up on the problem but there isn't really a solution.