At https://api.kde.org/frameworks/kdelibs4support/html/classKUrl.html#a04479511596e499f4f7ffa6acdc96cb3 there is documentated 

Deprecated:
    since 5.0, use QUrl::path(). ....

This advice results into code returning an invalid path on Windows in case QUrl contains a local path because QUrl::path() on Windows always adds a '/' in front of the path. 

The following example shows the issue 

    QUrl b = QUrl::fromLocalFile("/test/test.xyz");
    qDebug() << b << b.path() << b.toLocalFile();
    b = QUrl::fromLocalFile("c:/test/test.xyz");
    qDebug() << b << b.path() << b.toLocalFile();
    b = QUrl::fromLocalFile("c:\\test\\test.xyz");
    qDebug() << b << b.path() << b.toLocalFile() 

Compiling and running this on Windows (I used Qt 5.5) returns 

QUrl("file:///test/test.xyz") "/test/test.xyz" "/test/test.xyz"
QUrl("file:///c:/test/test.xyz") "/c:/test/test.xyz" "c:/test/test.xyz"
QUrl("file:///c:/test/test.xyz") "/c:/test/test.xyz" "c:/test/test.xyz"

The second column in line 2 and 3 shows the issue. 

Because KUrl::path() deals with this case by checking the "local file case" and using toLocalFile() instead porting to QUrl::path() directly will fail therefore.
Also the hint to use url.adjusted(QUrl::StripTrailingSlash).path() in case the option RemoveTrailingSlash has been used does not help here.

There should be a related hint into the documentation.