Why potentially dangerous request error even ValidateRequest = false

In .net framework 4.0 you have to set <httpRuntime requestValidationMode="2.0"/> markup in web.config.

<system.web>
     <compilation debug="false" targetFramework="4.0" />
     <httpRuntime requestValidationMode="2.0"/>
</system.web>

Have a look at reference article - ASP.NET 4 Breaking Changes #1: requestValidationMode cause ValidateRequest=False to fail.