Comment # 12 on bug 1204864 from Andreas

(In reply to Fabian Vogt from comment #11)
> (In reply to Andreas from comment #10)
> > (In reply to Fabian Vogt from comment #9)
> > > (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.
> > 
> > qdbus-qt5 org.freedesktop.impl.portal.desktop.kde
> > /
> > /org
> > /org/freedesktop
> > /org/freedesktop/portal
> > /org/freedesktop/portal/desktop
> > 
> > It is the same without a issue and it doesn't fail
> >  with issue
> 
> That is very strange, so the service is running correctly.

Maybe it's this bug that was fixed in Plasma 5.24.5?

xdgactivation: Demand attention when a process fails to resolve its token 
https://invent.kde.org/plasma/kwin/-/commit/5390a4978f3a3903ede15ecd94a750b37931fbd2