[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.]
This is a feature of IIS to avoid tampering with the viewstate and cookies. The viewstate of Page X is posted to Page Y and fails the Message Authentication Code (MAC)
We don’t need the viewstate so we disable it. But even with EnableViewstate = “false” the server generated the __VIEWSTATE hidden field.
var vs = document.getElementById("__VIEWSTATE"); vs.parentNode.removeChild(vs);
Code from jberda answer on ASP.NET forum.
Disabling viewstate is not recommended. (Details) Use it wisely.