This bug is not fixed in 5.2.0:
https://bugreports.qt-project.org/browse/QTBUG-32314

Simply try this, it will fail:
Q_ASSERT(QDir().mkpath(QLatin1String("C:
Users")));

It is easy to track the problem: the Qt implementation relies on GetLastError, but this Microsoft API has a bug, as mentionned there:
http://stackoverflow.com/questions/3570618/what-causes-createdirectory-to-return-error-access-denied

Hence the Qt implementation fails.
Qt code:
qtbase\src\corelib\io\qfilesystemengine_win.cpp
function:
QFileSystemEngine::createDirectory

I suggest to revert to the previous Qt implementation.