The error means that the references to the assembly are not correct. I advise that you remove them again, copy over the correct versions (from the Bin40 folder for .NET 4.0 for example) and make sure that all our assemblies are the same version. Then, just add references to them in the project. Generally, when upgrading a project I would advise that you follow these guidelines
You also need to look for Register directives (in pages, user controls or in the web.config) that use the fully qualified assembly name, because there may be a mistake in one of them that can be causing such a problem during compilation.
You may also find this post useful because it treats VS assembly references in some more details: http://blogs.telerik.com/aspnet-ajax/posts/13-09-19/working-with-assemblies-in-the-global-assembly-cache
The RadTreeview.Net2.dll is a very old assembly from the RadControls for ASP Classic line that has been discontinued for quite some time. I advise that you remove it.
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