>>Don't know if it could be the problem but you could check that the website itself is targeting .NET 4 (Properties/ASP.NET: ASP.NET version
>
>The application pool is v4.0.
>
>Under the site itself, I cannot find a related setting for that. Note that this is a virtual directory on my PC. So, it goes like http://localhost/LevelExtreme20. Is there an ASP.NET setting at the Web site level itself, so to check what you are making reference to, on a virtual directory?

Not sure what version of IIS I'm looking at at the moment but the only place I see to set the .NET framework version is, as above, from the Web site properties on the ASP.NET tab. Don't see any similar option for an Application pool at all ?
I also don't think you can vary the .NET version for a virtual directory - it's fixed at the Website level....