The ID of QStandardItemModel changes on every launch:
plasma-discover
libs QList("/usr/lib/x86_64-linux-gnu/qt6/plugins", "/usr/bin")
org.kde.plasma.libdiscover: OdrsReviewsBackend: Fetch ratings: true
adding empty sources model QStandardItemModel(0x5de8ff8ece80)
^C

I'm seeing this too, very consistently.  It appears to be the same as #481993, but that one has been marked fixed (albeit with a hack).  

As in that bug, plasma-discover hangs if I do not specify any backends. But it succeeds if I specify all the backends.  But the order matters.

        $ plasma-discover                                             
        libs QList("/usr/lib/x86_64-linux-gnu/qt6/plugins", "/usr/bin")
        org.kde.plasma.libdiscover: OdrsReviewsBackend: Fetch ratings: false
        adding empty sources model QStandardItemModel(0x5bf3e89f78c0)
        ^C


When I specified the backends to load and included all of them, it worked.

        $ plasma-discover --listbackends
        Available backends:
        * packagekit-backend
        * snap-backend
        * fwupd-backend
        * flatpak-backend
        * kns-backend

        $ plasma-discover --backends packagekit,fwupd,kns,snap,flatpak
        org.kde.plasma.libdiscover: OdrsReviewsBackend: Fetch ratings: false
        adding empty sources model QStandardItemModel(0x5a420195d800)
        qrc:/qt/qml/org/kde/kirigami/dialogs/Dialog.qml:344:18: QML ScrollView: Binding loop detected for property "calculatedImplicitWidth"
        qrc:/qt/qml/org/kde/kirigami/dialogs/Dialog.qml:344:18: QML ScrollView: Binding loop detected for property "calculatedImplicitHeight"
        qrc:/qt/qml/org/kde/kirigami/dialogs/Dialog.qml:401:33: QML Binding: Binding loop detected for property "value"
        qrc:/qt/qml/org/kde/discover/qml/BrowsingPage.qml:17:1: QML BrowsingPage: Created graphical object was not placed in the graphics scene.


If I specify the backends in the order they were listed, it fails the same as before:

        $ plasma-discover --backends packagekit,snap,fwupd,flatpak,kns
        org.kde.plasma.libdiscover: OdrsReviewsBackend: Fetch ratings: false
        adding empty sources model QStandardItemModel(0x62a8c78bd620)
        ^C

Same behavior after upgrade to 24.04 LTS, does not load. 
Running from command line does not open anything or shows any errors

Actually, it shows:
$ plasma-discover
libs QList("/usr/lib/x86_64-linux-gnu/qt6/plugins", "/usr/bin")
org.kde.plasma.libdiscover: OdrsReviewsBackend: Fetch ratings: true
adding empty sources model QStandardItemModel(0x62d3a8610650)

Launching second time does not show any errors, but keeps hanging until CTRL+C.

OK, this is funny (or not). I have installed the debug symbols:
$ pkcon install plasma-discover-dbgsym
Resolving                               [=========================]
Testing changes                         [=========================]
Finished                                [                         ] (0%)
The following packages have to be installed:
plasma-discover-dbgsym-6.2.3-0zneon+24.04+noble+release+build8.amd64   debug symbols for plasma-discover
Proceed with changes? [N/y] y

[=========================]
Installing                              [=========================]
Waiting for authentication              [=========================]
Loading cache                           [=========================]
Downloading packages                    [=========================]
Running                                 [=========================]
Finished                                [=========================]

Then suddenly Discover started. Until I closed it again and now I am unable to start it again...

Created attachment 175933 [details]
strace log

strace log

as confirmed by other users:
https://www.reddit.com/r/kde/comments/1b5ludx/discover_doesnt_start_up_on_plasma_6_in_kde_neon/

OK, related to:
https://bugs.kde.org/show_bug.cgi?id=481993
or possibly a duplicate.

Solution recommended here works:
https://bugs.kde.org/show_bug.cgi?id=481993#c4

these work for me when running from command line:
plasma-discover --backends packagekit-backend
plasma-discover --backends flatpak-backend
plasma-discover --backends packagekit-backend,flatpak-backend

Since this bug is not resolved in the last KDE-Neon Jammy with Plasma 6.2.0, is there any fix available?
We have many users, who can't start Discover after upgrading from Plasma 6.1.5 to Plasma 6.2.0 last available versions in neons jammy repo.

Running from Plasma systray, I have to kill it first:
$ pkill discover

Then running it from CLI shows these issues:
$ plasma-discover
libs QList("/usr/lib/x86_64-linux-gnu/qt6/plugins", "/usr/bin")
org.kde.plasma.libdiscover: OdrsReviewsBackend: Fetch ratings: false
adding empty sources model QStandardItemModel(0x59e70bd83160)

Killing it and running again with backends specified works:
$ plasma-discover --backends packagekit-backend,flatpak-backend
org.kde.plasma.libdiscover: OdrsReviewsBackend: Fetch ratings: true
adding empty sources model QStandardItemModel(0x5e5b75dd8750)
qrc:/qt/qml/org/kde/discover/qml/BrowsingPage.qml:17:1: QML BrowsingPage: Created graphical object was not placed in the graphics scene.
qrc:/qt/qml/org/kde/discover/qml/UpdatesPage.qml:11:1: QML UpdatesPage: Created graphical object was not placed in the graphics scene.
qrc:/qt/qml/org/kde/discover/qml/UpdatesPage.qml:40:5: QML OverlaySheet: Binding loop detected for property "y":
file:///usr/lib/x86_64-linux-gnu/qt6/qml/org/kde/kirigami/templates/OverlaySheet.qml:110:5

(In reply to Torsten Wohlfarth (TUXEDO) from comment #9)
> Since this bug is not resolved in the last KDE-Neon Jammy with Plasma 6.2.0,
> is there any fix available?
> We have many users, who can't start Discover after upgrading from Plasma
> 6.1.5 to Plasma 6.2.0 last available versions in neons jammy repo.

hiya.  is this still a thing?  just found an old jammy iso from july, installed it, updated it to the latest our jammy archives have to offer.  discover starts first-time, every time \o/ \o/

above the user states ^^^ that they have upgraded to noble 24.04 and still the issue persists.  we have no other bug reports of note about this in noble.  

smells like an old config or deprecated package problem.

After the latest (system) update in KDE Neon, Discover started after clicking on it in systray. I'll have to repeat this to confirm it works every time. So it does not seem to be an old config but rather some messed update.

Operating System: KDE neon 6.2
KDE Plasma Version: 6.2.4
KDE Frameworks Version: 6.8.0
Qt Version: 6.8.1
Kernel Version: 6.8.0-50-generic (64-bit)
Graphics Platform: X11

(In reply to Daniel Duris from comment #12)
> After the latest (system) update in KDE Neon, Discover started after
> clicking on it in systray. I'll have to repeat this to confirm it works
> every time. So it does not seem to be an old config but rather some messed
> update.
glad to hear it works for you.  just curious, are you running TUXEDO OS or a pure KDE neon install?

(In reply to Carlos De Maine from comment #13)
> (In reply to Daniel Duris from comment #12)
> > After the latest (system) update in KDE Neon, Discover started after
> > clicking on it in systray. I'll have to repeat this to confirm it works
> > every time. So it does not seem to be an old config but rather some messed
> > update.
> glad to hear it works for you.  just curious, are you running TUXEDO OS or a
> pure KDE neon install?

Unfortunately not working anymore. I suspect it worked after the upgrade and reboot, but now it's not working again (after closing it) and I have to run it via CLI again.

This is a KDE Neon install.

have noticed that sometimes i need to run killall plasma-discover as there are zombie processes hanging around and the gui doesn't want to show.  not sure if that is a neon specific problem or a bug in the source.

Can somebody please push a fix? This is clearly trivial to fix as the solution described here works:
https://ambience.sk/workaround-discover-does-not-launch-in-plasma-6-2-kde-neon/

But it is annoying that you can not properly launch Discover via systray notifier / GUI.