asp.net unhandled exception

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.System.Web   at System.Web.UI.ViewStateException.ThrowError(Exception inner, String persistedState, String errorPageMessage, Boolean macValidationError)     at System.Web.UI.ObjectStateFormatter.Deserialize(String inputString, Purpose purpose)     at System.Web.UI.ObjectStateFormatter.System.Web.UI.IStateFormatter2.Deserialize(String serializedState, Purpose purpose)     at System.Web.UI.Util.DeserializeWithAssert(IStateFormatter2 formatter, String serializedState, Purpose purpose)     at System.Web.UI.HiddenFieldPageStatePersister.Load()     at System.Web.UI.Page.LoadPageStateFromPersistenceMedium()     at System.Web.UI.Page.LoadAllState()     at System.Web.UI.Page.Pr ...