Validation of viewstate MAC failed.Application hosted by a Web Farm, ensure configuration [duplicate]

Answer 1

you can add your machine key in system.web element. for e.g.

<system.web>
<machineKey validationKey="1234567890123456789012345678901234567890AAAAAAAAAA"
 decryptionKey="123456789012345678901234567890123456789012345678"
 validation="SHA1" decryption="Auto" />
</system.web>

If you don't want to put a machinekey in the web.config for each application, you need to put it in machine level web.config for each .Net Framework version.