Validation of viewstate MAC failed?

[T111]

I have a webpage with a filter invoked by a javascript function -

<script type="text/javascript">
<!--
var theForm = document.forms['form1'];
if (!theForm) {
    theForm = document.form1;
}
function __doPostBack(eventTarget, eventArgument) {
    if (!theForm.onsubmit || (theForm.onsubmit() != false)) {
        theForm.__EVENTTARGET.value = eventTarget;
        theForm.__EVENTARGUMENT.value = eventArgument;
        theForm.submit();
    }
}
// -->
</script>

And here is the code when the filter is invoked -

<select name="ddOffice" onchange="javascript:setTimeout('__doPostBack(\'ddOffice\',\'\')', 0)" id="ddOffice" style="font-weight:bold;width:152px;">
	<option selected="selected" value="%%">Department</option>
	<option value="Sales">Sales</option>
	<option value="Technical">Technical</option>
</select>

Anyone know how to solve this, I added

<pages validateRequest="false" maxPageStateFieldLength="1024" enableViewStateMac="false" enableEventValidation="false" viewStateEncryptionMode ="Never"   >

to the webconfig and added

<meta http-equiv="imagetoolbar" content="RevealTrans(Duration=0,Transition=0)" />

to the top of the page but still no luck.Any help appreciated,T111!

 

[T111]

It seems to be a bug in VWD 2005, as the table in question had 18 records but when I removed most of them the form started working again. I decided to use a gridview instead of a formview.