Asp.net The anti-forgery token could not be decrypted hatası

Windows 2008 Server IIS 7.5 sunucumda Event Viewer içerisinde,  Asp.Net MVC web uygulamama ait aşağıdakine benzer uyarılar(warning) görüyorum.

Event message: An unhandled exception has occurred. 
Event Id: 1309
Process name: w3wp.exe 

Exception information: 
    Exception type: HttpAntiForgeryException 
    Exception message: The anti-forgery token could not be decrypted. If this application is hosted by a Web Farm or cluster, ensure that all machines are running the same version of ASP.NET Web Pages and that the <machineKey> configuration specifies explicit encryption and validation keys. AutoGenerate cannot be used in a cluster.

"The anti-forgery token could not be decrypted." hatalarının sebebi nedir?