Application failed to start because it could not find or load the QT platform plugin “windows”

The error is caused because the program can't find qwindows.dll

qwindows.dll has to be in a folder named platforms so that the path from your executable to the dll is platforms/qwindows.dll

Whereas this wasn't enough in my case. I had also to add following line at the beginning of my main()

QCoreApplication::addLibraryPath("./");

Then everything worked.