1. Make sure that there is any combobox in any app expanded: e.g. in Krusader expand the combobox showing the currently browsed path; or in Chromium hit Ctrl+S and in the save-as dialog expand the Filter: combobox 2. Now press the PrintScreen/PrtSc button on the keyboard. Nothing happens. 3. However, when you press Esc to collapse the combobox options, PrtSc button starts working correctly.
Ubuntu 17.04 x64 fully updated; Plasma 5.9.4; QT 5.7.1, KDE frameworks 5.31.0
*** This bug has been marked as a duplicate of bug 70063 ***