Could not load file or assembly 'AjaxControlToolkit' or one of its dependencies. The process cannot access the file because it is being used by another process. (Exception from HRESULT: 0x80070020)
You get this message if you try to replace a dll in the \bin directory while refreshing your aspx page.
Solution: wait until the dll files are uploaded before loading the aspx page.
Entire error message: