Solution:The viewstate is invalid for this page and might be corrupted.

This error comes up in web farms when a user downloads the webpage from one server and responds to a different server. By default, each webserver generates it's own encryption keys for the viewstate; if the user does a postback to a different server, it won't be able to decrypt the viewstate.

The solution is to hard code the encryption/decription MachineKeys in the machine.config file for all the servers in the webfarm.