Do you experience the described behavior on a public page or in edit mode? Please, make sure you have referenced your custom DLL in the ~/bin
folder of the web site. If everything is properly set, you need to debug the code in the custom project and localize what exactly causes the issue. You could also remove the reference and the related code to be sure that it is the reason for the problem.
Do let us know how it goes.
the Telerik team
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