Could not load file or assembly 'AjaxControlToolkit, Version=1.0.10301.0, Culture=neutral, PublicKeyToken=28f01b0e84b6d53e' or one of its dependencies. Access Denied (Zugriff verweigert)

Review the permissions of the user that you're trying to impersonate.

In my situation, I was only getting the error on my development machine, rather than on our staging or deployment servers. (For while I got around this by removing the 'identity' node from web.config in my dev environment and just adding the line in post-build so it wasn't a problem anyone other than me.