I hate this error. No seriously, I HATE this error. It happens from time to time, usually when trying to use any 3rd-party components in your Visual Studio toolbox. That means if you are using the Ajax Control Toolkit, Telerik controls, or any other set of components that didn’t come as part of the Visual Studio installation, you will probably see this message at one point or another. This error has popped up in numerous blogs and forum posts and there’s a list of solutions (and possible causes) which seems to work for most folks. Well, the bug still rears its ugly head for me from time to time and I have yet to find a definitive explanation for what causes it.

Tonight I encountered this error once again while trying to drag a Telerik control onto my ASP.NET page.  I was able to eliminate the error by resetting my Visual Studio Toolbox. If you run across this error, you might try the same by right-clicking anywhere in the toolbox and selecting Reset Toolbox from the context menu. Of course, now all your controls are not available in Visual Studio for rapid drag-and-drop development. But you can easily re-add them. Right-click anywhere in the toolbox, select Add Tab, and give the new tab a descriptive name. The quickest way to get controls into the new toolbox tab is to simply drag the relevant assembly into the tab. Alternatively, you can right-click in the toolbox and select Choose Items… to select an assembly and import its controls, but this method is slow.

I hope this helps.