Answer
Close down every open IFS EE window and all browser windows first (mandatory), then fully delete that Windows user's ClickOnce application cache directory at C:\Users\<username>\AppData\Local\Apps\2.0. Equivalently, after closing everything, you can run rundll32 dfshim CleanOnlineAppCache from the Windows Run dialog to clear the same ClickOnce cache. A corrupted local ClickOnce cache for one specific Windows profile — while every other user profile on the same or different machines works fine — is a common cause of this single-user-only 'missing DLL' symptom.