(In reply to Daniel Noga from comment #0)
> Discover reports "Unable to load applications. Please verify Internet
> connectivity". It reports error, even Firefox properly works and YaST
> software installation too.

What does the journal say? Does "pkcon refresh" work?

$ pkcon refresh
Refreshing cache              [=========================]         
Starting                      [=========================]         
Refreshing software list      [=========================]         
Finished                      [=========================]       




$ sudo journalctl | grep discover
Apr 10 12:33:53 localhost.localdomain plasma-discover[2084]: invalid kns backend! "/usr/share/knsrcfiles/ksysguard.knsrc" because: "Config group not found! Check your KNS3 installation."
Apr 10 12:33:53 localhost.localdomain plasma-discover[2084]: kf.newstuff.core: Your configuration file uses an old version of the kpackage support, and should be converted. Please report this to the author of the software you are currently using. The package type, we assume, is "KWin/Effect"
Apr 10 12:33:53 localhost.localdomain plasma-discover[2084]: kf.newstuff.core: Your configuration file uses an old version of the kpackage support, and should be converted. Please report this to the author of the software you are currently using. The package type, we assume, is "KWin/Script"
Apr 10 12:33:53 localhost.localdomain plasma-discover[2084]: kf.newstuff.core: Your configuration file uses an old version of the kpackage support, and should be converted. Please report this to the author of the software you are currently using. The package type, we assume, is "KWin/WindowSwitcher"
Apr 10 12:33:53 localhost.localdomain plasma-discover[2084]: kf.newstuff.core: Your configuration file uses an old version of the kpackage support, and should be converted. Please report this to the author of the software you are currently using. The package type, we assume, is "Plasma/Comic"
Apr 10 12:33:53 localhost.localdomain plasma-discover[2084]: kf.newstuff.core: Your configuration file uses an old version of the kpackage support, and should be converted. Please report this to the author of the software you are currently using. The package type, we assume, is "Plasma/Wallpaper"
Apr 10 12:33:53 localhost.localdomain plasma-discover[2084]: kf.newstuff.core: Your configuration file uses an old version of the kpackage support, and should be converted. Please report this to the author of the software you are currently using. The package type, we assume, is "Plasma/LookAndFeel"
Apr 10 12:33:53 localhost.localdomain plasma-discover[2084]: kf.newstuff.core: Your configuration file uses an old version of the kpackage support, and should be converted. Please report this to the author of the software you are currently using. The package type, we assume, is "Plasma/LookAndFeel"
Apr 10 12:33:53 localhost.localdomain plasma-discover[2084]: kf.newstuff.core: Your configuration file uses an old version of the kpackage support, and should be converted. Please report this to the author of the software you are currently using. The package type, we assume, is "Plasma/Theme"
Apr 10 12:33:53 localhost.localdomain dbus-daemon[1630]: [session uid=1000 pid=1630] Activating via systemd: service name='org.gtk.vfs.Daemon' unit='gvfs-daemon.service' requested by ':1.35' (uid=1000 pid=2084 comm="/usr/bin/plasma-discover ")
Apr 10 12:33:53 localhost.localdomain dbus-daemon[867]: [system] Activating via systemd: service name='org.freedesktop.fwupd' unit='fwupd.service' requested by ':1.52' (uid=1000 pid=2084 comm="/usr/bin/plasma-discover ")
Apr 10 12:33:53 localhost.localdomain dbus-daemon[1630]: [session uid=1000 pid=1630] Activating service name='org.kde.kwalletd5' requested by ':1.34' (uid=1000 pid=2084 comm="/usr/bin/plasma-discover ")
Apr 10 12:33:54 localhost.localdomain dbus-daemon[867]: [system] Activating via systemd: service name='org.freedesktop.PackageKit' unit='packagekit.service' requested by ':1.50' (uid=1000 pid=2084 comm="/usr/bin/plasma-discover ")
Apr 10 12:33:54 localhost.localdomain plasma-discover[2084]: adding empty sources model QStandardItemModel(0x5650a2c9ae40)
Apr 10 12:33:54 localhost.localdomain plasma-discover[2084]: org.kde.plasma.libdiscover: Couldn't find a category for  "fwupd-backend"
Apr 10 12:33:54 localhost.localdomain plasma-discover[2084]: file:///usr/lib64/qt5/qml/org/kde/kirigami.2/private/PrivateActionToolButton.qml:77:5: QML Binding: Binding loop detected for property "value"
Apr 10 12:33:57 localhost.localdomain plasma-discover[2084]: took really long to fetch PackageKitBackend(0x5650a2cc7f80)
Apr 10 12:34:49 localhost.localdomain plasma-discover[2084]: PackageKit stopped running!

(In reply to Daniel Noga from comment #0)
> 1) Do clean KDE Installation in Virtualbox via
> openSUSE-Leap-15.3-DVD-x86_64-Build117.13-Media.iso
> 2) Run Discover
> 
> Discover reports "Unable to load applications. Please verify Internet
> connectivity". It reports error, even Firefox properly works and YaST
> software installation too.

Same here~
And when lunching plasma-discover in Terminal.
Lots of errors were shown as following:

invalid kns backend! "/usr/share/knsrcfiles/ksysguard.knsrc" because: "Config group not found! Check your KNS3 installation."
kf.newstuff.core: Your configuration file uses an old version of the kpackage support, and should be converted. Please report this to the author of the software you are currently using. The package type, we assume, is "KWin/Effect"
kf.newstuff.core: Your configuration file uses an old version of the kpackage support, and should be converted. Please report this to the author of the software you are currently using. The package type, we assume, is "KWin/Script"
kf.newstuff.core: Your configuration file uses an old version of the kpackage support, and should be converted. Please report this to the author of the software you are currently using. The package type, we assume, is "KWin/WindowSwitcher"
kf.newstuff.core: Your configuration file uses an old version of the kpackage support, and should be converted. Please report this to the author of the software you are currently using. The package type, we assume, is "Plasma/Wallpaper"
kf.newstuff.core: Your configuration file uses an old version of the kpackage support, and should be converted. Please report this to the author of the software you are currently using. The package type, we assume, is "Plasma/Comic"
kf.newstuff.core: Your configuration file uses an old version of the kpackage support, and should be converted. Please report this to the author of the software you are currently using. The package type, we assume, is "Plasma/LookAndFeel"
kf.newstuff.core: Your configuration file uses an old version of the kpackage support, and should be converted. Please report this to the author of the software you are currently using. The package type, we assume, is "Plasma/LookAndFeel"
kf.newstuff.core: Your configuration file uses an old version of the kpackage support, and should be converted. Please report this to the author of the software you are currently using. The package type, we assume, is "Plasma/Theme"
adding empty sources model QStandardItemModel(0x55e7fec42a40)
org.kde.plasma.libdiscover: Couldn't find a category for  "fwupd-backend"
file:///usr/lib64/qt5/qml/org/kde/kirigami.2/private/PrivateActionToolButton.qml:77:5: QML Binding: Binding loop detected for property "value"

** (process:10254): WARNING **: 20:09:02.022: Found icon of unknown type, skipping it: unknown
remote hasn't changed: lvfs
took really long to fetch KNSBackend(0x55e7feb4be80, name = "/usr/share/knsrcfiles/sddmtheme.knsrc")
took really long to fetch KNSBackend(0x55e7fe4f9260, name = "/usr/share/knsrcfiles/konsole.knsrc")
took really long to fetch KNSBackend(0x55e7feb823c0, name = "/usr/share/knsrcfiles/aurorae.knsrc")
took really long to fetch KNSBackend(0x55e7feb90000, name = "/usr/share/knsrcfiles/kwineffect.knsrc")
took really long to fetch KNSBackend(0x55e7feb8a030, name = "/usr/share/knsrcfiles/kwinscripts.knsrc")
took really long to fetch KNSBackend(0x55e7feb876b0, name = "/usr/share/knsrcfiles/kwinswitcher.knsrc")
took really long to fetch KNSBackend(0x55e7feb908b0, name = "/usr/share/knsrcfiles/window-decorations.knsrc")
took really long to fetch KNSBackend(0x55e7feb879f0, name = "/usr/share/knsrcfiles/plasmoids.knsrc")
took really long to fetch KNSBackend(0x55e7feb967e0, name = "/usr/share/knsrcfiles/wallpaper.knsrc")
took really long to fetch KNSBackend(0x55e7feb95280, name = "/usr/share/knsrcfiles/wallpaperplugin.knsrc")
took really long to fetch KNSBackend(0x55e7feba54e0, name = "/usr/share/knsrcfiles/comic.knsrc")
took really long to fetch KNSBackend(0x55e7feb95d90, name = "/usr/share/knsrcfiles/colorschemes.knsrc")
took really long to fetch KNSBackend(0x55e7feb9ffb0, name = "/usr/share/knsrcfiles/emoticons.knsrc")
took really long to fetch KNSBackend(0x55e7febabe10, name = "/usr/share/knsrcfiles/gtk2_themes.knsrc")
took really long to fetch KNSBackend(0x55e7febb3950, name = "/usr/share/knsrcfiles/gtk3_themes.knsrc")
took really long to fetch KNSBackend(0x55e7feba2270, name = "/usr/share/knsrcfiles/icons.knsrc")
took really long to fetch KNSBackend(0x55e7febc3400, name = "/usr/share/knsrcfiles/ksplash.knsrc")
took really long to fetch KNSBackend(0x55e7febbdaf0, name = "/usr/share/knsrcfiles/lookandfeel.knsrc")
took really long to fetch KNSBackend(0x55e7febc5750, name = "/usr/share/knsrcfiles/plasma-themes.knsrc")
took really long to fetch KNSBackend(0x55e7febb7120, name = "/usr/share/knsrcfiles/xcursor.knsrc")
took really long to fetch KNSBackend(0x55e7febc88b0, name = "/usr/share/knsrcfiles/ksieve_script.knsrc")
took really long to fetch KNSBackend(0x55e7febc03b0, name = "/usr/share/knsrcfiles/accountwizard.knsrc")
took really long to fetch KNSBackend(0x55e7febc9ed0, name = "/usr/share/knsrcfiles/messageviewer_header_themes.knsrc")
took really long to fetch KNSBackend(0x55e7febc9b00, name = "/usr/share/knsrcfiles/knotes_printing_theme.knsrc")
took really long to fetch KNSBackend(0x55e7febc4a10, name = "/usr/share/knsrcfiles/korganizer.knsrc")
took really long to fetch KNSBackend(0x55e7febd1360, name = "/usr/share/knsrcfiles/discover_ktexteditor_codesnippets_core.knsrc")
took really long to fetch PackageKitBackend(0x55e7fec0db20)

I have the same problem in Opensuse Tumbleweed with Discover.

Work fine now in this configuration and snapshot

Sistema operativo: openSUSE Tumbleweed 20210623
Versión de KDE Plasma: 5.22.1
Versión de KDE Frameworks: 5.83.0
Versión de Qt: 5.15.2
Versión del kernel: 5.12.12-1-default (64 bits)
Plataforma gráfica: X11
Procesadores: 4 × AMD A10-7700K APU with Radeon™ R7 Graphics
Memoria: 15,1 GiB de RAM
Procesador gráfico: AMD KAVERI

I just tested in latest Leap 15.3 and latest Tumbleweed and bug appears only on Leap. I am switching affected version back to Leap 15.3

So far it looked like it was just a symptom of store.kde.org having some issues, but if it only happens on TW that's a bit strange.

Does it work after uninstalling discover-backend-packagekit?

It only happens on Leap 15.3. Not on Leap 15.2, not on Tumbleweed. On tumbleweed is only one bug report from Vernius, which isn't valid anymore, according to second Verniuse's comment.

Bug is easily reproducable on clean installation.

No, removing discover-backend-packagekit or discover-backend-flatpak or discover-backend-fwupd does not help.

Same here~~ In fact, I had removed the discover and all its dependencies

The following 34 packages are going to be REMOVED:
  discover discover-backend-flatpak discover-backend-fwupd discover-backend-packagekit discover-lang efibootmgr flatpak fwupd fwupd-lang gnome-desktop-lang grub2-arm64-efi
  libflatpak0 libfwupd2 libfwupdplugin1 libgnome-desktop-3_0-common libgnome-desktop-3-18 libgusb2 libjcat1 libostree libostree-1-1 libsmbios_c2 libtss2-esys0 libtss2-mu0
  libtss2-sys0 libxmlb2 shim system-user-flatpak typelib-1_0-Fwupd-2_0 xdg-desktop-portal xdg-desktop-portal-gtk xdg-desktop-portal-gtk-lang xdg-desktop-portal-kde
  xdg-desktop-portal-kde-lang xdg-desktop-portal-lang


and reinstalled the discover. Problem remains....

The problem is still present in Leap 15.3. Allegedly, it's fixed in Tumbleweed at least for a month according to the discussion here: https://forums.opensuse.org/showthread.php/555475-Discover

I've got fully updated installation. pkcon refresh doesn't fix it, neither does removing discover-backend-packagekit.

david@atronach-opensuse:~> plasma-discover
invalid kns backend! "/usr/share/knsrcfiles/ksysguard.knsrc" because: "Config group not found! Check your KNS3 installation."
kf.newstuff.core: Your configuration file uses an old version of the kpackage support, and should be converted. Please report this to the author of the software you are currently using. The package type, we assume, is "Plasma/Comic"
kf.newstuff.core: Your configuration file uses an old version of the kpackage support, and should be converted. Please report this to the author of the software you are currently using. The package type, we assume, is "Plasma/Theme"
kf.newstuff.core: Your configuration file uses an old version of the kpackage support, and should be converted. Please report this to the author of the software you are currently using. The package type, we assume, is "KWin/Script"
kf.newstuff.core: Your configuration file uses an old version of the kpackage support, and should be converted. Please report this to the author of the software you are currently using. The package type, we assume, is "KWin/WindowSwitcher"
kf.newstuff.core: Your configuration file uses an old version of the kpackage support, and should be converted. Please report this to the author of the software you are currently using. The package type, we assume, is "Plasma/LookAndFeel"
kf.newstuff.core: Your configuration file uses an old version of the kpackage support, and should be converted. Please report this to the author of the software you are currently using. The package type, we assume, is "Plasma/Wallpaper"
kf.newstuff.core: Your configuration file uses an old version of the kpackage support, and should be converted. Please report this to the author of the software you are currently using. The package type, we assume, is "Plasma/LookAndFeel"
kf.newstuff.core: Your configuration file uses an old version of the kpackage support, and should be converted. Please report this to the author of the software you are currently using. The package type, we assume, is "KWin/Effect"
adding empty sources model QStandardItemModel(0x55b0da83b490)
org.kde.plasma.libdiscover: Couldn't find a category for  "fwupd-backend"
file:///usr/lib64/qt5/qml/org/kde/kirigami.2/private/PrivateActionToolButton.qml:77:5: QML Binding: Binding loop detected for property "value"

** (process:17383): WARNING **: 04:03:25.556: Found icon of unknown type, skipping it: unknown
remote hasn't changed: lvfs
took really long to fetch KNSBackend(0x55b0da817110, name = "/usr/share/knsrcfiles/korganizer.knsrc")
took really long to fetch KNSBackend(0x55b0da825630, name = "/usr/share/knsrcfiles/xcursor.knsrc")
took really long to fetch KNSBackend(0x55b0da8150b0, name = "/usr/share/knsrcfiles/aurorae.knsrc")
took really long to fetch KNSBackend(0x55b0da80eeb0, name = "/usr/share/knsrcfiles/sddmtheme.knsrc")
took really long to fetch KNSBackend(0x55b0da828a50, name = "/usr/share/knsrcfiles/comic.knsrc")
took really long to fetch KNSBackend(0x55b0da844360, name = "/usr/share/knsrcfiles/discover_ktexteditor_codesnippets_core.knsrc")
took really long to fetch KNSBackend(0x55b0da835e60, name = "/usr/share/knsrcfiles/plasma-themes.knsrc")
took really long to fetch KNSBackend(0x55b0da8311a0, name = "/usr/share/knsrcfiles/kwinscripts.knsrc")
took really long to fetch KNSBackend(0x55b0da816be0, name = "/usr/share/knsrcfiles/kwinswitcher.knsrc")
took really long to fetch KNSBackend(0x55b0da820100, name = "/usr/share/knsrcfiles/icons.knsrc")
took really long to fetch KNSBackend(0x55b0da836cf0, name = "/usr/share/knsrcfiles/wallpaper.knsrc")
took really long to fetch KNSBackend(0x55b0da84ff50, name = "/usr/share/knsrcfiles/emoticons.knsrc")
took really long to fetch KNSBackend(0x55b0da84f480, name = "/usr/share/knsrcfiles/lookandfeel.knsrc")
took really long to fetch KNSBackend(0x55b0da850e50, name = "/usr/share/knsrcfiles/wallpaperplugin.knsrc")
took really long to fetch KNSBackend(0x55b0da835140, name = "/usr/share/knsrcfiles/konsole.knsrc")
took really long to fetch KNSBackend(0x55b0da83e120, name = "/usr/share/knsrcfiles/ksieve_script.knsrc")
took really long to fetch KNSBackend(0x55b0da83ecd0, name = "/usr/share/knsrcfiles/ksplash.knsrc")
took really long to fetch KNSBackend(0x55b0da8649b0, name = "/usr/share/knsrcfiles/gtk3_themes.knsrc")
took really long to fetch KNSBackend(0x55b0da8390c0, name = "/usr/share/knsrcfiles/gtk2_themes.knsrc")
took really long to fetch KNSBackend(0x55b0da8545e0, name = "/usr/share/knsrcfiles/kwineffect.knsrc")
took really long to fetch PackageKitBackend(0x55b0da7d4a50)

Apparently the cause of this is boo#1191245.

Although that report is newer, it contains more relevant information, so let's close this as a duplicate.

*** This bug has been marked as a duplicate of bug 1191245 ***