Answer
This kind of odd, machine-specific glitch in the ClickOnce-based IEE client is commonly resolved by clearing that machine's local Online Application Cache: close every open IFS window on the affected machine first, open a Command Prompt, and run rundll32 dfshim CleanOnlineAppCache. The next time IFS is launched on that machine, you'll see a 'Downloading feature pack' re-download prompt, after which this kind of glitch is commonly resolved, since the command clears a stale or corrupted local ClickOnce deployment cache specific to that one machine.