I just had the same problem and I solved it by updating the Newtonsoft.Json to the latest version using
Update-Package Newtonsoft.Json
and then going to Web.config and adding:
<dependentAssembly>
<assemblyIdentity name="Newtonsoft.Json" publicKeyToken="30ad4fe6b2a6aeed"/>
<bindingRedirect oldVersion="0.0.0.0-4.5.0.0" newVersion="5.0.8"/>
</dependentAssembly>