This seems to be general .NET issue that sometimes occurs when you are using web farm or cluster. Could you please try to change your web.config file as described at
support.aspdotnetstorefront.com/index.php You could also eventually disable viewstate MAC validation by setting enableViewStateMac property of <page> element to false.
You can put machineKey setting in your web.config. Here is an example:
<machineKey validationKey="AutoGenerate,IsolateApps" decryptionKey="AutoGenerate,IsolateApps" validation="SHA1" />
But you will need to insert your own keys instead of '
AutoGenerate' setting. Or you can disable viewstate MAC validation by setting
enableViewStateMac property of
<page> element to false. You can find more information at:
http://msdn2.microsoft.com/en-us/library/950xf363.aspx
http://msdn2.microsoft.com/en-us/library/w8h3skw9.aspx
http://aspadvice.com/blogs/joteke/archive/2006/02/02/15011.aspx
and another great article:
http://docs.orchardproject.net/Documentation/Setting-up-a-machine-key
As a temporary fix, you can also add the following key to your web.config file:
<add key="CMSUseViewStateUserKey" value="false"/>
This should only be seen as a temporary solution, as it is not recommended to have this key set to false, as having this set to false reduces your site’s security.
If you are using version 6.0, there were some problems in earlier hotfix versions that could cause this if a user’s IP changed during a single session. Updating to the latest hotfix version may resolve this issue.