For anyone coming from QT version 5.14.0, it took me 2 days to find this piece statment of bug:
windeployqt does not work for MinGW QTBUG-80763 Will be fixed in 5.14.1
https://wiki.qt.io/Qt_5.14.0_Known_Issues
So be aware. Using windeployqt withMinGW will give the same error stated here.