The reason for this error is validation of Viewstate failing because of different value of key for encryption/decryption across different server on farm. Below link provide good explanation of this error and possible resolution for the same.
Add these lines in web.config
<system.web>
<pages enableViewStateMac="false" />
</system.web>
OR
Add Machine key on web.cofing
sample machine key
<machinekey>
validationKey="9B648362BE0A4C3FEDD4D9C97F148CA71497C5128BF543DDB37D70AEFBA2F16D184A900A
C1C16331F5E95708BAD6A67286B74CEBBE4ACDEA58B787AF1BD7BFE4"
decryptionKey="86D46880FD9C2F391D9176A24EF22A16560AD69B7AE37146"
validation="SHA1" />
You need to add this to encrypt the ViewState information is a WebFarm
Reference:
http://msdn.microsoft.com/en-us/library/ms998288.aspx
Please read the above reference from MSDN you will come to know clearly