If you enable the "Use FIPS compliant algorithms for encryption, hashing, and signing" security policy option in Windows, attempting to use many of the cryptographic classes in the .NET Framework will result in an InvalidOperationException. By default, ASP.NET uses AES to encrypt the ViewState blob, so it fails. You can work around this by adding a key like this to web.config:If you enable the "Use FIPS compliant algorithm