The described behaviour is probably result of the fact you are using Firefox 6.x to create the new library. If that is the case, I will explain why you experience the issue - Firefox 6 introduced some key changes in its core functionality which broke Sitefinity. We have released a patch which fixes the problem, but it can be applied only to Sitefinity 4.2 versions. You can download it from your account downloads page and apply it to your project.
If you keep getting the error using other browsers as well please provide me with more information about the project so I can get a better picture of your situation.
the Telerik team
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