System.Web.HttpException: Unable to validate data

We have an ASP.Net MVC3 web application hosted on IIS behind a load balancer. We have the persistence session set to 2 hours on the load balancer so users' requests don't switch between servers. Users are still getting the below error. I read about adding the same machine key in the web.conig file on both servers. After setting a persistence session set to 2 hours why this issue is still happening? Any ideas to fix this issue?

System.Web.HttpException: Unable to validate data.

System.Web.HttpException (0x80004005): Unable to validate data.
at System.Web.Configuration.MachineKeySection.EncryptOrDecryptData(Boolean fEncrypt, Byte[] buf, Byte[] modifier, Int32 start, Int32 length, Boolean useValidationSymAlgo, Boolean useLegacyMode, IVType ivType, Boolean signData)
at System.Web.Security.FormsAuthentication.Decrypt(String encryptedTicket)
at App.Security.Model.Authentication.AppFormsAuthentication.GetCurrentAuthenticationTicket()