I tried to reproduce the problem, but to no avail.
Currently I can only make assumptions why this issue is encountered. You could try the following:
- Check if all references are with correct paths to the GAC or to the folder they used to refer;
- Check if the Web application's target framework is set to 4.0 in its properties and accordingly in the web.config file;
- Is the problem reproducible with the built-in ASP.NET controls? If it is then this might be more related to a Visual Studio configuration problem;
- Are there differences in the application settings or the configuration between VS2010 and VS2012?
If the problem appears only with our controls it would be helpful if you could provide a simple locally runnable project, which isolates the problematic field, so that I could investigate this on my end.
Note that the errors pointed out in your message differ from the ones in the provided snapshot. Could you please determine which are the thrown errors and are they thrown in the browser's dev toolbar or the Visual Studio's output view?
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