Comment # 13 on bug 1100765 from Wolfgang Bauer

(In reply to . . from comment #12)
> When running qdbus-qt5 org.kde.KWin /Screenshot:
> ....
> Error: org.freedesktop.DBus.Error.UnknownObject
> No such object path '/Screenshot'
> ....

Is compositing disabled? Does it help if you enable it?

Compositing is mandatory for kwin's screenshot interface to work, as otherwise
kwin has no idea about the windows' contents.