I did this quick hack and it worked, see if its the same for you:
- Remove the Tab and controls from toolbox in visual studio.
- Add a new tab for holding Ajax toolkit controls
- Load toolkit controls by browing the ajaxcontroltoolit.dll (make sure you select the version you want to load) and fill the required tab with controls
- Now when you will drag and drop the controls on aspx form, it will not show up previous error