Timeout Exception in ASP.net

Question

I have developed an web application and published the same on web server.

 

When i open the application, and keep it idle for some time say 5-10 minutes then try to click on some option. It throws an exception as given below. Can anybody please help me get out of this. 

 

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.

 

Thanks in advance.

-Satinder