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.