This one is a bit weird. It happens on my client, whether run from the View client on a PC or from a zero client - Windows Explorer, IE, and Administrative Tools windows have no visible minimize, maximize or close buttons, although hovering over where they should be in the upper right-hand corner of the window frame brings up the name of the button for that position and clicking where the button should be visible performs the function of that button's location. I have tried other themes, like high contrast, and they are still not visible.
I have searched on Google for the problem, but haven't seen anyone else with the issue (yet). MS Office applications and most other applications show the buttons normally. I have attached a screen shot of the corner of one of the windows for reference.