Are you using anything like any type of custom Windows themes, theme tweaks, start menu replacements or Aero Glass mod type of things? Perhaps also things like WindowBlinds could break this too.
No, since installation of Windows 10 I use very few third party system tools as I want to keep my system fluid that time.
BUT: I found the reason. Most the time I start MC using a batch file which modifies the registry and copies some files to customize MC for different environments (
Dance school, mobile Laptop use, docking station with three soundcards). Using this batch starts MC as administrator and: taskbar controls won't work. Starting MC as "normal" user makes them work as expected. I had the same batch files with windows 8(.1) and now problems using the taskbar controls.
MC bug, Windows 10 bug or just some weired security feature?
P.S.: How to start a file as normal user out of an admin batch?