Asp.Net Error Message in Event Viewer:Unable to validate data

Question

We have a Asp.Net Webform page which contains a GridView inside UpdatePanel and refreshes every minute. And every one minute we get the following error in Event log.

Error Message:Unable to validate data.
Stack Trace:   at System.Web.Configuration.MachineKeySection.GetDecodedData(Byte[] buf, Byte[] modifier, Int32 start, Int32 length, Int32& dataLength)
   at System.Web.UI.ObjectStateFormatter.Deserialize(String inputString).

We have tried the following.

1. Adding a static machine key in the Web.Config. (Did not work?)
2. Disabling the View State Mac in the Web,.Config using following entry. (Did not work)

<pages buffer="true" enableViewStateMac="false">

Is there something else that might cause this?

Update: We get this error only on the Live Server. Not on Test or Dev server.