I am not sure what the actual problem is here. Let's try the following
1. Run the project through VS web server- this will eliminate issues that comes from IIS.
2. If the problem persists, create a new template and try to open it. It is possible that there is something on your master page that breaks the backend page editor. Do not add scripts or css classes to the newly create template.
3. When you use IIS, please make sure that the application pool that the upgraded project uses is set to use ASP.NET 4.0.
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