Search our database viewstate error:

If you get the following error message:

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.

It is due to some bugs in .Net framework, simply add in the following line in web.config under httpdocs, in the <system.web> section: