Comment # 3 on bug 1165646 from Stefan Hundhammer

When I look at that screenshot, it appears to me that there is no window
manager running: There is just the pure Qt window, no window manager
decorations (no WM borders, no window title, no window buttons, nothing).

Since this does not seem to affect all installations, I think it is safe to
assume that the window manager is on the installation media in principle and
that it normally works; but it might have crashed in this scenario (or never
started at all).

It is hard to guess from outside of that machine why that might have happened.