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.

Hi i get the above error whenever i click on a button in my application, this has occured ever since I have started using urlrewriting, does anyone know what may be the cause of this, this is an example of one of my urlrewrites just incase it gives a clue;
<rewrite url="/choices/" to Go to the complete details ...