Error:-
Could not load file or assembly 'AjaxControlToolkit' or one of its dependencies. The parameter is incorrect. (Exception from HRESULT: 0x80070057 (E_INVALIDARG))
Personal Note:- This occurred in my ASP.NET web application
Solution:- 
Delete all or related files inside the folder 
C:\Windows\Microsoft.NET\Framework\v2.0.50727\Temporary ASP.NET Files