The fix for
QTBUG-30046 (change 3e2cd8ef6f1ce4f46719890134c8bba9dbdb19ba) introduced a regression on Windows: depending on the current working directory, QDir::mkdir for an absolute path might fail.
Analysis: The patch splits up the path, and calls ::CreateDirectory for every part. For absolute paths it starts with the drive letter, e.g. "D:". However, before calling CreateDirectory we call QFSFileEnginePrivate::longFileName(path), which is supposed to prefix a \\?\. Anyhow, since "D:" isn't a complete path it also tries to add the working directory at the end, if the working directory is on the same drive.
If the working directory is on another drive, ::CreateDirectory is called with "\\?\D:\". This path, however, results in an windows error 5 - Access is denied.