Assert or corrupt layout when opening window on 2nd monitor

When opening (a rather complex) window in our (Windows) application, while the application is currently positioned on a screen that is not the main screen:

• When linked against Qt debug libraries there will be two asserts:

Program: D:\VisualStudio\2015_______\bin64d\Qt5Cored.dll
Module: 5.9.0
File: C:\dev\qt\qtbase\src\corelib\global\qglobal.cpp
Line: 3044

ASSERT: "win == this->window() || this->window()->isAncestorOf(win, QWindow::ExcludeTransients)" in file C:\dev\qt\qtbase\src\gui\painting\qbackingstore.cpp, line 115

• When linked against the release libraries, the window will show up but it's layout is distorted.

The errors do not occur when opening the window while the application is running on the main screen.

The errors also do not occur on the second screen if the window has been opened and closed on the main screen once.