After some useless technical advices from the support team of Prometric (from India, I think), I finally figured out the problem.
It has nothing to do with enabling rotation or hotkeys in the Graphic properties (rightclick menu at desktop), but it is the refresh rate in there. Someone must have changed it as it was different than others'. So I simply set it back to 60 hertz from 75. And bingo, no more upside down screen when the full test screen is loaded.