Bug 413699

Here are app logs:
Icon theme "elementary" not found.
Icon theme "gnome" not found.
Icon theme "elementary" not found.
Icon theme "gnome" not found.
adding empty sources model QStandardItemModel(0x55e2cbf65c60)
no packages for "org.kde.plasma.quicklaunch"
no packages for "org.kde.plasma.weather"
no packages for "org.kde.plasma.systemmonitor.memory"
no packages for "org.kde.latte.shell"
no packages for "org.kde.desktopcontainment"
no packages for "org.kde.plasma.networkmanagement"
no packages for "org.kde.plasma.systemloadviewer"
no packages for "org.kde.plasma.webbrowser"
no packages for "org.kde.plasma.clipboard"
no packages for "de.urwpp.D050000L"
no packages for "org.kde.plasma.pager"
no packages for "de.urwpp.URWBookman"
no packages for "org.kde.plasmoidviewer"
no packages for "org.kde.plasma.showActivityManager"
no packages for "de.urwpp.StandardSymbolsPS"
no packages for "org.kde.plasma.notes"
no packages for "org.kde.plasma.trash"
no packages for "de.urwpp.URWCoreFontSetLevel2"
no packages for "org.kde.plasma.comic"
no packages for "org.kde.plasma.windowlist"
no packages for "org.kde.plasma.battery"
no packages for "org.kde.hunyango"
no packages for "org.kde.haenau"
no packages for "org.kde.plasma.lock_logout"
no packages for "org.kde.plasma.keyboardindicator"
no packages for "org.kde.paneltoolbox"
no packages for "org.kde.plasma.grouping"
no packages for "org.kde.plasma.analogclock"
no packages for "org.kde.plasma.folder"
no packages for "org.kde.color"
no packages for "org.kde.image"
no packages for "de.urwpp.P052"
no packages for "org.kde.plasma.bluetooth"
no packages for "de.urwpp.C059"
no packages for "org.kde.plasma.systemmonitor.diskusage"
no packages for "org.kde.plasma.showdesktop"
no packages for "org.kde.plasma.diskquota"
no packages for "gimp-data-extras"
no packages for "org.kde.plasma.themeexplorer"
no packages for "org.kde.plasma.systemtray"
no packages for "de.urwpp.NimbusMonoPS"
no packages for "org.kde.plasma.calendar"
no packages for "org.kde.plasma.lookandfeelexplorer"
no packages for "org.gnome.cantarell"
no packages for "org.kde.kdialog"
no packages for "org.kde.plasma.systemmonitor.diskactivity"
no packages for "org.freedesktop.appstream.cli"
no packages for "org.kde.breezedark.desktop"
no packages for "org.kde.plasma.userswitcher"
no packages for "org.kde.plasma.systemmonitor.net"
no packages for "de.urwpp.NimbusRoman"
no packages for "org.kde.plasma.desktop.emptyPanel"
no packages for "org.kde.desktoptoolbox"
no packages for "org.kde.plasma.fuzzyclock"
no packages for "org.kde.plasma.kicker"
no packages for "org.kde.plasma_applet_dict"
no packages for "org.kde.plasma.activitypager"
no packages for "org.kde.plasma.colorpicker"
no packages for "org.kde.plasma.cuttlefish"
no packages for "org.kde.plasmashell"
no packages for "org.kde.plasma.fifteenpuzzle"
no packages for "org.kde.plasma.timer"
no packages for "org.kde.kscreen"
no packages for "org.kde.latte.plasmoid"
no packages for "org.kde.milou"
no packages for "org.kde.plasma.mediacontroller"
no packages for "org.kde.plasma.konsoleprofiles"
no packages for "org.kde.discover.packagekit"
no packages for "org.kde.breeze.desktop"
no packages for "org.kde.plasma.quickshare"
no packages for "org.kde.plasma.mediaframe"
no packages for "org.kde.plasma.systemmonitor.cpu"
no packages for "de.urwpp.Z003"
no packages for "org.kde.plasma.activitybar"
no packages for "org.kde.potd"
no packages for "org.kde.slideshow"
no packages for "org.kde.plasma.icontasks"
no packages for "org.kde.kinfocenter"
no packages for "org.kde.plasma.taskmanager"
no packages for "org.kde.plasma.digitalclock"
no packages for "org.kde.plasma.desktop.appmenubar"
no packages for "de.urwpp.URWGothic"
no packages for "org.kde.plasma.volume"
no packages for "org.kde.plasma.desktop.defaultPanel"
no packages for "org.kde.plasma.kickerdash"
no packages for "org.kde.plasma.calculator"
no packages for "org.kde.plasma.appmenu"
no packages for "org.kde.discover.flatpak"
no packages for "org.kde.plasmoidviewershell"
no packages for "org.kde.plasma.notifications"
no packages for "org.kde.plasma.devicenotifier"
no packages for "org.kde.plasma.binaryclock"
no packages for "org.kde.plasma.desktop"
no packages for "org.kde.plasma.kimpanel"
no packages for "de.urwpp.NimbusSans"
no packages for "org.kde.plasma.kickoff"
no packages for "org.kde.plasma.vault"
no packages for "org.kde.plasma.minimizeall"
empty appstream db
org.kde.plasma.libdiscover: error loading "fwupd-backend" "Cannot load library /usr/lib/qt/plugins/discover/fwupd-backend.so: (libfwupd.so.2: cannot open shared object file: No such file or directory)" QJsonObject({"IID":"org.kde.muon.AbstractResourcesBackendFactory","archreq":0,"className":"FwupdBackendFactory","debug":false,"version":331008})
org.kde.plasma.libdiscover: error loading "flatpak-backend" "Cannot load library /usr/lib/qt/plugins/discover/flatpak-backend.so: (libflatpak.so.0: cannot open shared object file: No such file or directory)" QJsonObject({"IID":"org.kde.muon.AbstractResourcesBackendFactory","archreq":0,"className":"FlatpakBackendFactory","debug":false,"version":331008})
invalid kns backend! "/usr/share/knsrcfiles/ksysguard.knsrc" because: "Config group not found! Check your KNS3 installation."

Just opened Discover today, it's working now.
Weird.