We encountered a problem with viewstate encryption on Windows 2008 R2 and Windows 7. The page in question does a post with javascript to provide default values for parameters for another page. Only on IIS 7.5 this generates the error 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.]

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.

No more mister nice guy! We added the javascript below in the post code to remove the viewstate. No more viewstate, no more exceptions.

TLDR

var vs = document.getElementById("__VIEWSTATE"); 
vs.parentNode.removeChild(vs);

Code from jberda answer on ASP.NET forum.

Caution

Disabling viewstate is not recommended. (Details) Use it wisely.