Validation of viewstate MAC failed.

Question

Hello friends,<br /> <br /><div> I have uploaded my web site on my domain it works fine for all features but when i load my view data page ,data i can see but if i leave that page idle for some time(20 min) and then refersh the page that time error is coming.</div><div>&nbsp;</div><div><strong>Server Error in Application.<br /> <br /> 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.</machinekey></strong></div><div>&nbsp;</div><div>suggest me some solution.</div><div>thank you!!!&nbsp;</div>