Error:System.Web.HttpException: 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.

Error:System.Web.HttpException: 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.

Cause:

Solution:Add this to page enableViewStateMac="false"

This option will work perfectly, but it is always advisable to go with specifying the custom key for encryption and decryption in web.config. 


http://forums.asp.net/t/1355367.aspx/1