Program menus do not open when window is on second screen

When moving a window of a program which uses QT (e.g. VLC, LyX, kdiff3) to my second screen, the program menus (from menu bar and right-click menus) do not open. When moving the window back to the main screen, the menus open again, but enlarged to full screen height at the first opening.
This happens on currently stable Debians 64bit, 32bit and Ubuntu 16.04.3. (All systems use on-board-graphic cards, at least the Debian ones Intel)
Please feel free to ask for all needed details.