The handle is invalid when loading file or assembly AjaxControlToolkit

I'm having one error repeatedly. The site is on ASP.NET 2.0 web form. There is no pattern to reproduce this error again because it occurs sometimes and it resolve by adding blank space at end of the in web.config.

What could be the problem?

Server Error in '/' Application. The handle is invalid. (Exception from HRESULT: 0x80070006 (E_HANDLE))

Exception Details: System.Runtime.InteropServices.COMException: The handle is invalid. (Exception from HRESULT: 0x80070006 (E_HANDLE))

[COMException (0x80070006): The handle is invalid. (Exception from HRESULT: 0x80070006 (E_HANDLE))]

[FileLoadException: Could not load file or assembly 'AjaxControlToolkit, Version=3.0.30930.28736, Culture=neutral, PublicKeyToken=28f01b0e84b6d53e' or one of its dependencies. The handle is invalid. (Exception from HRESULT: 0x80070006 (E_HANDLE))]

[ConfigurationErrorsException: Could not load file or assembly 'AjaxControlToolkit, Version=3.0.30930.28736, Culture=neutral, PublicKeyToken=28f01b0e84b6d53e' or one of its dependencies. The handle is invalid. (Exception from HRESULT: 0x80070006 (E_HANDLE))] System.Web.Configuration.CompilationSection.LoadAssemblyHelper(String assemblyName, Boolean starDirective) +613

AjaxControlToolkit, Version=3.0.30930.28736 is not compatible with asp.net 2.0

Note: The current version of the AJAX Control Toolkit is not compatible with ASP.NET 2.0. The latest version that is compatible with ASP.NET 2.0 can be found here: 20229

Remove current AjaxControlToolkit and install compatible version of it.