Sitefinity 4 "ExceptionManager Cannot be Constructed" error

If you're using Sitefinity 4.x and suddenly begin seeing this curious ASP.Net error:

"The type ExceptionManager cannot be constructed. You must configure the container to supply this value."

...this error results when the App_Data folder, and files within, are marked Read Only - if you're using TFS for source control, make sure your App_Data folder has been Checked Out. Your app should then launch normally.