ERROR FIXING FOR .NET WEBSITE

HII, We have a small website for online quiz. some error is coming at our website. ERROR DISCRIPTION is( see attachment for details): Server Error in '/' Application. ________________________________________ Validation of viewstate MAC failed. If this application is hosted by a Web Farm or cluster, ensure that configuration specifies the same validationKey and validation algorithm. AutoGenerate cannot be used in a cluster. Description: An unhandled exception occurred during the execution of the current web request. Please review the stack trace for more information about the error and where it originated in the code. Exception Details: [login to view URL]: Validation of viewstate MAC failed. If this application is hosted by a Web Farm or cluster, ensure that configuration specifies the same validationKey and validation algorithm. AutoGenerate cannot be used in a cluster. intrested service providers can bid for solving this error. thanks amit