If you get the following error in your ASP.NET application please follow the instructions below:
HttpException (0x80004005): 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.
First you need to generate a machine key. You can do that at the link below:
Once you have the machine key, copy and paste it into your web.config in the root of your application. It should be placed within the system.web tags.
There is more information on machine keys at the link below if you want to know more details on the subject: