We have a working Silverlight 3 project. After changing the target framework to Silverlight 4 the application won't start anymore. It throws an exception in the following line in the generated InitializeComponent() method:

System.Windows.Application.LoadComponent(this, new System.Uri("/SLAppMain;component/App.xaml", System.UriKind.Relative));

Here is the exception (note the inner exception):

System.Windows.Markup.XamlParseException occurred
  Message= [Line: 0 Position: 0]
  LineNumber=0
  LinePosition=0
  StackTrace:
       bei System.Windows.Application.LoadComponent(Object component, Uri resourceLocator)
  InnerException: System.TypeLoadException
       Message=Der Typ 'System.Security.AllowPartiallyTrustedCallersAttribute' konnte nicht aus der mscorlib, Version=2.0.5.0, Culture=neutral, PublicKeyToken=7cec85d7bea7798e -Assembly geladen werden.
       StackTrace:
            bei System.ModuleHandle.ResolveType(RuntimeModule module, Int32 typeToken, IntPtr* typeInstArgs, Int32 typeInstCount, IntPtr* methodInstArgs, Int32 methodInstCount, ObjectHandleOnStack type)
            bei System.ModuleHandle.ResolveTypeHandleInternal(RuntimeModule module, Int32 typeToken, RuntimeTypeHandle[] typeInstantiationContext, RuntimeTypeHandle[] methodInstantiationContext)
            bei System.Reflection.RuntimeModule.ResolveType(Int32 metadataToken, Type[] genericTypeArguments, Type[] genericMethodArguments)
            bei System.Reflection.CustomAttribute.FilterCustomAttributeRecord(CustomAttributeRecord caRecord, MetadataImport scope, Assembly& lastAptcaOkAssembly, RuntimeModule decoratedModule, MetadataToken decoratedToken, RuntimeType attributeFilterType, Boolean mustBeInheritable, Object[] attributes, IList derivedAttributes, RuntimeType& attributeType, IRuntimeMethodInfo& ctor, Boolean& ctorHasParameters, Boolean& isVarArg)
            bei System.Reflection.CustomAttribute.GetCustomAttributes(RuntimeModule decoratedModule, Int32 decoratedMetadataToken, Int32 pcaCount, RuntimeType attributeFilterType, Boolean mustBeInheritable, IList derivedAttributes, Boolean isDecoratedTargetSecurityTransparent)
            bei System.Reflection.CustomAttribute.GetCustomAttributes(RuntimeModule decoratedModule, Int32 decoratedMetadataToken, Int32 pcaCount, RuntimeType attributeFilterType, Boolean isDecoratedTargetSecurityTransparent)
            bei System.Reflection.CustomAttribute.GetCustomAttributes(RuntimeAssembly assembly, RuntimeType caType)
            bei System.Reflection.RuntimeAssembly.GetCustomAttributes(Type attributeType, Boolean inherit)
            bei System.Attribute.GetCustomAttributes(Assembly element, Type attributeType, Boolean inherit)
            bei MS.Internal.XamlSchemaContext.ProcessXmlnsDefinitions(Assembly assembly, String assemblyName)
            bei MS.Internal.XamlSchemaContext.EnsureManagedAssemblyAttributesLoaded()
       InnerException: 

The problem is that the type System.Security.AllowPartiallyTrustedCallersAttribute is not contained in the Silverlight version of mscorlib.dll.

I have no idea how to fix this nor where to look for causes. Has anyone encountered this problem? What could possibly cause this error?