Why am I getting the "A potentially dangerous Request.Form value was detected from the client" error?

I've created a new ASP.NET MVC 3 / .NET Framework 4.0 site using the "Internet Application" template. I used Nuget to install the Windows Azure Web Role (MVC3) package and then followed the Access Control Service walkthrough to set up Windows Live ID and Google authentication.

Soon enough, I came across the "A potentially dangerous Request.Form value was detected from the client" error and followed the article in the Windows Identity Foundation wiki to try and resolve it. Unfortunately nothing I've tried works, including:

• Setting <httpRuntime requestValidationMode="2.0"/> and <pages validateRequest="false"> in both the root web.config and Views\web.config

• Copying SampleRequestValidator from the WIF SDK into the project and setting <httpRuntime requestValidationType="SampleRequestValidator"/> in both web.configs

I've also tried variations of these without success.

Any ideas?

Here's the complete exception:

Exception Details: System.Web.HttpRequestValidationException: A potentially dangerous Request.Form value was detected from the client (wresult="<t:RequestSecurityTo...").

Description: Request Validation has detected a potentially dangerous client input value, and processing of the request has been aborted. This value may indicate an attempt to compromise the security of your application,