InputBox causes ViewState Validation failure

I am getting an error in my VB code: "Validation of viewstate MAC failed".  Note that I am NOT using a cluster environment, the viewstate mac failure seems to be a "catchall" error message for Microsoft, so the error message has no helpful information.

To see the error goto: http://www.thedesk.info/myplan/printerFriendlyTop2.aspx?domain=community&pageToPrint=print.aspx
then click "Email".  All of the other options work.

I have determined that by removing a call to InputBox, the error goes away.  Also, the error states that I can get more information by making debug="true", but it is both globally and in the page.

Does anyone know why a call to InputBox would cause this error and how I can get around it?  My web.config is as follows:

<?xml version="1.0" encoding="iso-8859-1"?>
<!-- Web.Config Configuration File -->

<configuration xmlns="http://schemas.microsoft.com/.NetConfiguration/v2.0">
    <system.web>
        <customErrors mode="Off"/>
                        <pages validateRequest="false" enableEventValidation="false" viewStateEncryptionMode ="Never" />
        <compilation debug="true"/>
          </system.web>
</configuration>

<%@ Page Language="VB"%>
<script language="VB" runat="Server">
 
...
 
  Sub SendEmail(ByVal sender As Object, ByVal e As System.EventArgs)
    Dim email As String = InputBox("Please enter your email address")
    Dim pageToLoad As String = "print.aspx?domain=" & domain.Value & "&userName=" & userName.Value
    Dim tx As New System.IO.StringWriter
    Server.Execute(pageToLoad, tx)
	  
    Dim ct As System.Net.Mime.ContentType = New System.Net.Mime.ContentType(System.Net.Mime.MediaTypeNames.Text.Html)
    Dim plan As Net.Mail.Attachment = Net.Mail.Attachment.CreateAttachmentFromString(tx.ToString(), ct)
    plan.ContentDisposition.FileName = "myplan_" & domain.Value & ".html"
    plan.ContentDisposition.DispositionType = System.Net.Mime.MediaTypeNames.Text.Html
		
    Dim msg As String = "Your plan is attached"
    GeneralFunctionsVB.mail(email, "MyPlan", "myplan@thedesk.info", msg, Nothing, Nothing, 0, plan)
  End Sub
</script>
 
...
 
<!-- The .NET code for handling the Email link -->
<asp:LinkButton ID="EmailLink" runat="server" CssClass="actionLink" OnClick="SendEmail">Email...</asp:LinkButton>

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