Comment # 9 on bug 1204864 from Fabian Vogt

(In reply to Andreas from comment #7)
> (In reply to Fabian Vogt from comment #6)
> > (In reply to Andreas from comment #5)
> > > (In reply to Fabian Vogt from comment #4)
> > > > Display :99 sounds strange. Which display manager do you use? If it's
> > > > lightdm, try sddm or gdm.
> > > I'm using the default SDDM for KDE Plasma installation.
> > 
> > So the question is where DISPLAY=:99 is coming from. IIRC that's what
> > Flatpak sets inside the sandbox for X11 mediation. I have no idea what might
> > send that into the dbus activation environment.
> 
> > I have the suspicion that flatpak breaks this somehow. Can you try the
> > following (I don't have access to 15.4 myself currently):
> I tried to reproduce this on a Gnome Live CD and there is no such problem.
> 
> > 1. qdbus-qt5 org.freedesktop.impl.portal.desktop.kde (to start the process
> > without using flatpak)
> I start it with sudo

Don't run any of the commands with sudo.