Sitecore 6.6 to 7.0 upgrade - sitecore

Question

Upgrading from sitecore 6.6.0 (rev. 130529) to 7 and all worked fine until I tried to access 'Content Editor'. I get this strange error and have no idea what need to be done.
Exception: System.Web.HttpUnhandledException
Message: Exception of type 'System.Web.HttpUnhandledException' was thrown.
Source: System.Web
at System.Web.UI.Page.HandleError(Exception e)
at System.Web.UI.Page.ProcessRequestMain(Boolean includeStagesBeforeAsyncPoint, Boolean includeStagesAfterAsyncPoint)
at System.Web.UI.Page.ProcessRequest(Boolean includeStagesBeforeAsyncPoint, Boolean includeStagesAfterAsyncPoint)
at System.Web.UI.Page.ProcessRequest()
at System.Web.UI.Page.ProcessRequestWithNoAssert(HttpContext context)
at System.Web.UI.Page.ProcessRequest(HttpContext context)
at ASP.sitecore_shell_applications_content_manager_default_aspx.ProcessRequest(HttpContext context) in c:\Windows\Microsoft.NET\Framework\v4.0.30319\Temporary ASP.NET Files\root\953a4a3d\d5c79dc4\App_Web_l41k3wf4.2.cs:line 0
at System.Web.HttpApplication.CallHandlerExecutionStep.System.Web.HttpApplication.IExecutionStep.Execute()
at System.Web.HttpApplication.ExecuteStep(IExecutionStep step, Boolean& completedSynchronously)
Nested Exception
Exception: System.Reflection.TargetInvocationException
Message: Exception has been thrown by the target of an invocation.
Source: mscorlib
at System.RuntimeMethodHandle.InvokeMethod(Object target, Object[] arguments, Signature sig, Boolean constructor)
at System.Reflection.RuntimeMethodInfo.UnsafeInvokeInternal(Object obj, Object[] parameters, Object[] arguments)
at System.Reflection.RuntimeMethodInfo.Invoke(Object obj, BindingFlags invokeAttr, Binder binder, Object[] parameters, CultureInfo culture)
at Sitecore.Reflection.ReflectionUtil.CallMethod(Type type, Object obj, String methodName, Boolean includeNonPublic, Boolean includeInherited, Boolean includeStatic, Object[] parameters)
at Sitecore.Shell.Applications.ContentManager.ContentEditorPage.OnPreRender(EventArgs e)
at System.Web.UI.Control.PreRenderRecursiveInternal()
at System.Web.UI.Page.ProcessRequestMain(Boolean includeStagesBeforeAsyncPoint, Boolean includeStagesAfterAsyncPoint)
Nested Exception
Exception: System.TypeLoadException
Message: Access is denied: 'Sitecore.Shell.Applications.ContentEditor.RenderingDatasource'.
Source: mscorlib
at System.Reflection.RuntimeAssembly.GetType(RuntimeAssembly assembly, String name, Boolean throwOnError, Boolean ignoreCase, ObjectHandleOnStack type)
at System.Reflection.RuntimeAssembly.GetType(String name, Boolean throwOnError, Boolean ignoreCase)
at Sitecore.Web.UI.XmlControls.ControlSource.ResolveType(String controlName, Assembly assembly)
at Sitecore.Web.UI.XmlControls.AssemblyControlSource.ResolveType(String controlName, Boolean includeDynamicTypes)
at Sitecore.Web.UI.XmlControls.ControlSource.GetControlType(String namespacePrefix, String controlName, String mode, Boolean includeDynamicTypes)
at Sitecore.Web.UI.XmlControls.ControlFactory.GetControlType(String controlName, String mode, Boolean includeDynamicTypes)
at Sitecore.Web.UI.XmlControls.ControlFactory.GetControl(Control parent, String controlName, String mode, Object[] properties, AllowGenericControl allowGeneric)
at Sitecore.Shell.Applications.ContentEditor.EditorFormatter.GetEditor(Item fieldType)
at Sitecore.Shell.Applications.ContentEditor.EditorFormatter.RenderField(Control parent, Field field, Item fieldType, Boolean readOnly, String value)
at Sitecore.Shell.Applications.ContentEditor.EditorFormatter.RenderField(Control parent, Field field, Item fieldType, Boolean readOnly)
at Sitecore.Shell.Applications.ContentEditor.EditorFormatter.RenderField(Control parent, Field field, Boolean readOnly)
at Sitecore.Shell.Applications.ContentEditor.EditorFormatter.RenderSection(Section section, Control parent, Boolean readOnly)
at Sitecore.Shell.Applications.ContentEditor.EditorFormatter.RenderSections(Control parent, Sections sections, Boolean readOnly)
at Sitecore.Shell.Applications.ContentEditor.Pipelines.RenderContentEditor.RenderStandardContentEditor.Process(RenderContentEditorArgs args)
at (Object , Object[] )
at Sitecore.Pipelines.PipelineMethod.Invoke(Object[] parameters)
at Sitecore.Pipelines.CorePipeline.Run(PipelineArgs args)
at Sitecore.Pipelines.CorePipeline.Run(String pipelineName, PipelineArgs args, String pipelineDomain, Boolean failIfNotExists)
at Sitecore.Pipelines.CorePipeline.Run(String pipelineName, PipelineArgs args, String pipelineDomain)
at Sitecore.Pipelines.CorePipeline.Run(String pipelineName, PipelineArgs args)
at Sitecore.Shell.Applications.ContentManager.Editor.Render(RenderContentEditorArgs args, Control parent)
at Sitecore.Shell.Applications.ContentManager.Editor.Render(Item item, Item root, Hashtable fieldInfo, Control parent, Boolean showEditor)
at Sitecore.Shell.Applications.ContentManager.ContentEditorForm.RenderEditor(Item item, Item root, Control parent, Boolean showEditor)
at Sitecore.Shell.Applications.ContentManager.ContentEditorForm.UpdateEditor(Item folder, Item root, Boolean showEditor)
at Sitecore.Shell.Applications.ContentManager.ContentEditorForm.Update()
at Sitecore.Shell.Applications.ContentManager.ContentEditorForm.OnPreRendered(EventArgs e)
Tried few times but I cannot see this going away. Anyone else had this before?
Updated with full stack now. I'm not sure if first 2 exceptions has anything to do with this but it might be part of the problem.

Answer 1

The error is from Sitecore trying to load an assembly via reflection and getting a system level access denied on the DLL. You should check that the identity your AppPool runs as has access to the webroot, in particular, the /bin folder and all it's contents.

Answer 2

Go the IIS and select the the site App pool. Then click on the Advance Setting and navigate to the Process Model Section. Change the Identity to NetworkService.
On your website folder, right click and go to properties. Select the Security Tab and add the following user:
Everyone - check all checkboxes to provide full access
Also, make sure that NETWORK SERVICE is present in the list of usernames which all required access: