Updated 28.10.2011: Fixed the link

We have an application which uses the lovely portable class libraries to share classes between a Silverlight and an ASP.NET MVC application. Everything went fine until the application was deployed to a test server, at which point the ASP.NET MVC site stopped working with the following error:

Could not load file or assembly 'System.Xml.Serialization”.

Our ASP.NET MVC makes a call to a WebService and the required proxy classes are inside a portable class library project. Turns out that solving this issue was very easy once we knew where to look at. You just need to install a .NET Framework 4 patch, the KB2468871 to be more precise. After we installed the update, everything just worked. And the reason we didn’t encounter this problem with out development environments was because the VS 2010 SP1 includes this patch.