Loading this assembly would produce a different grant set from other instances

[EDN Admin]

Not sure what I did on this one.. to me however, I have never seen this issue before. Today I downloaded .NET 4.5 and installed. The funny thing is that VS2010 didnt show 4.5 under Project Properties. All of my files in this older solution
I was needing to change came up a read protected. The cs files would allow me to overwrite them. I went to the main folder and unchecked everything from READ ONLY. Now I see this whenever trying to bring up XAML pages in the editor.
I dont know if this is related to the fact I once checked this solution into TFS and attempted to edit my local copy, or something else Ive SNAFUd.
Please advise.
<pre class="prettyprint <span style="font-size:x-small System.IO.FileLoadException
Loading this assembly would produce a different grant set from other instances. (Exception from HRESULT: 0x80131401)
at System.RuntimeTypeHandle.CreateInstance(RuntimeType type, Boolean publicOnly, Boolean noCheck, Boolean& canBeCached, RuntimeMethodHandleInternal& ctor, Boolean& bNeedSecurityCheck)
at System.RuntimeType.CreateInstanceSlow(Boolean publicOnly, Boolean skipCheckThis, Boolean fillCache, StackCrawlMark& stackMark)
at System.RuntimeType.CreateInstanceDefaultCtor(Boolean publicOnly, Boolean skipCheckThis, Boolean fillCache, StackCrawlMark& stackMark)
at System.Activator.CreateInstance(Type type, Boolean nonPublic)
at System.RuntimeType.CreateInstanceImpl(BindingFlags bindingAttr, Binder binder, Object[] args, CultureInfo culture, Object[] activationAttributes, StackCrawlMark& stackMark)
at System.Activator.CreateInstance(Type type, BindingFlags bindingAttr, Binder binder, Object[] args, CultureInfo culture, Object[] activationAttributes)
at System.Activator.CreateInstance(String assemblyString, String typeName, Boolean ignoreCase, BindingFlags bindingAttr, Binder binder, Object[] args, CultureInfo culture, Object[] activationAttributes, Evidence securityInfo, StackCrawlMark& stackMark)
at System.Activator.CreateInstance(String assemblyName, String typeName)
at System.AppDomain.CreateInstance(String assemblyName, String typeName)
at System.AppDomain.CreateInstanceAndUnwrap(String assemblyName, String typeName)
at System.AppDomain.CreateInstanceAndUnwrap(String assemblyName, String typeName)
at MS.Internal.Package.VSIsolationProviderService.CreateIsolationProvider(String originalIdentifier, Boolean isGlobal, String identity, FrameworkName frameworkName, AssemblyName appAssemblyName, IVsHierarchy hierarchy)
at MS.Internal.Package.VSIsolationProviderService.CreateIsolationProviderWorker(String identifier, IServiceProvider provider)
at MS.Internal.Package.VSIsolationProviderService.CreateIsolationProvider(String identifier, IServiceProvider provider)
at MS.Internal.Providers.VSDesignerContext.CreateIsolationProvider(IServiceProvider provider, IVsHierarchy hierarchy)
at MS.Internal.Providers.VSDesignerContext.<>c__DisplayClass1.<Initialize>b__0(IsolationProviderProxy i)
at MS.Internal.Providers.IsolationProviderProxy.get_RealProvider()
at MS.Internal.Providers.IsolationProviderProxy.add_UnhandledException(UnhandledExceptionEventHandler value)
at MS.Internal.Designer.DesignerPane.LoadDesignerView(Boolean isReload) [/code]
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