While working on the current project I needed to use some controls from the ajax control toolkit, so I added a reference to it's dll file to the project and to the Toolbox. Unfortunatelly, when I wanted to drag some controls to the page, I had messagebox with the error in the topic. What was strange, in other new website project, there everything worked ok. After a few minutes on google, I found some solutions: link In my case unload and load again a project (without restarting VS) with UI helped ......