When deploying the QT 5 application, after adding all (as far as I thought) required DLLs, you might get an error like this:
Failed to load platform plugin "windows". Available platforms are: minimal, offscreen, windows
In documentation (https://qt-project.org/doc/qt-5.0/qtdoc/deployment-windows.html) it stands that one should put platforms/qwindows.dll in the app directory, but it didn't work in my case. Actually, I've found out something weird, that it's enough to add the libEGL.dll in the app directory, and that works. I'm not a QT developer I must be honest, so I don't know why it helped, maybe it's ok. But nothing like this was said in the documentation, so I decided to write it here, maybe someone will be looking for the same problem and now google can help him (it didn't help in my case ;)).