Replaced HttpUtility.UrlEncode with Uri.EscapeDataString and deleted the reference to the System.Web assembly. The System.Web assembly seems to be missing on some .NET 4.0 installations (and the overhead of using it is a bit large, just for the UrlEncode method).

 <?xml version="1.0" encoding="utf-8"?>

 <configuration>

   <startup>

     <supportedRuntime version="v4.0" />

     <supportedRuntime version="v2.0.50727"/>

   </startup>

 </configuration>