ASP.NET is not able to load types of custom web events if they are placed under App_Code directory. Most probably it happens because Type.GetType() method is used, which cannot load ASP.NET 2.0 App_Code types since they are available during execution time only. We should use System.Web.Compilation.BuildManager.GetType() method instead. For now the solution is only to separate needed type to another assembly and point configuration to it.