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Getting "Validation of viewstate MAC failed" only with IE on some machines

HI, I have a web application that is used by the company I work for. It is running on a Windows 2008 r2 server with IIS 7. I just started getting this error on one web page today, only on two users machines that I know of (mine included), it works on all otheres I have checked. I only get the error when using IE 9, I get no error if I hit the same page from Firefox.
The only clue I have is that server updates were performed this weekend, service packs installed, etc. I tried clearing out my cache. I'm stumped, any ideas?


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.

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AkAlan
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AkAlan
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Easwaran ParamasivamCommented:
In your web.config file have below changes.

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

Refer the thread: http://forums.asp.net/t/1355367.aspx/1

if does not work update the machine key in the web.config file

http://www.shnake.com/blog/?p=604
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AkAlanAuthor Commented:
EaswaranP, Thanks for your reply. I think I have a couple of things going on that contributed to the exception happening on just a couple of machines. I was looking into getting faster page loads a couple of weeks back and ended up putting this code in the Page Load event of the page in question.
  'Response.Cache.SetExpires(DateTime.Now.AddMonths(1))
   'Response.Cache.SetCacheability(HttpCacheability.ServerAndPrivate)
   'Response.Cache.SetValidUntilExpires(True)
I had forgot about it until today when I was still getting the exception.
I remarked it out and after clearing the users cache, no more exception.
I am using Telerik controls and I need to have viewstate enable for the gridview on the page, at least I'm pretty sure of that.
Not being a real asp.net expert, I am not certain why I got the exception on only a couple of machines so if any of this makes sense to you and you can give me any further guidance, I would appreciate it. I will give you the points either way. thanks for your assistance.  
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