Validation of viewstate MAC failed. If this application is hosted by a Web Farm or cluster, ensure that
same validationKey and validation algorithm. AutoGenerate cannot be used in a cluster.
This error may occur when one hosts an ASP.NET webpage on a web-garden.
Simple solution (for ASP.NET 2.0 and up): Generate a new machinekey and add it to the web.config.
Full explanation can be found in the blog of Jody.