I was trying to add AJAX Control Toolkit to my Toolbox, for some reason Visual Studio was crashing. I found a solution for it, see this works out for you – start Visual Studio in safe mode (go to Visual Studio’s command prompt and execute ‘devenv /safemode’) and then try to add items to your toolbox, I was able to do it! There might be a better solution for this, please let me know if there is one.
Disclaimer The opinions expressed herein are my own personal opinions and do not represent my employer's view in any way.