SUMMARY
This happened after updating an Arch Linux system. I'll keep an eye on this and close it if the issue disappears due to fixes for the Qt 6.7 packaging.

STEPS TO REPRODUCE
1. Open system settings.

OBSERVED RESULT
None of the KCMs display (of the ones I checked). In the app, I see errors like:

Error loading QML file.

qrc:/kcm/kcm_landingpage/main.qml:18 Type KCM.SimpleKCM unavailable
qrc:/qt/qml/org/kde/kcmutils/SimpleKCM.qml:33 Type Kirigami.ScrollablePage unavailable
file:///usr/lib/qt6/qml/org/kde/kirigami/ScrollablePage.qml:68 Type Kirigami.Page unavailable
file:///usr/lib/qt6/qml/org/kde/kirigami/Page.qml:237 Type Kirigami.PageRow unavailable
file:///usr/lib/qt6/qml/org/kde/kirigami/PageRow.qml:180 Type OverlayDrawer unavailable
file:///usr/lib/qt6/qml/org/kde/kirigami/OverlayDrawer.qml:22 Type KT.OverlayDrawer unavailable
file:///usr/lib/qt6/qml/org/kde/kirigami/templates/OverlayDrawer.qml:120 Cannot assign object of type "KTP.IconPropertiesGroup" to property of type "IconPropertiesGroup_QMLTYPE_52*" as the former is neither the same as the latter nor a sub-class of it.

And in the terminal I see this output:

KQuickPadding is both anonymous and named
qt.qml.typeresolution.cycle: Cyclic dependency detected between "qrc:/qt/qml/org/kde/desktop/private/GlobalSonnetSettings.qml" and "qrc:/qt/qml/org/kde/desktop/private/MobileTextActionsToolBar.qml"
qt.qml.typeresolution.cycle: Cyclic dependency detected between "qrc:/qt/qml/org/kde/desktop/private/GlobalSonnetSettings.qml" and "qrc:/qt/qml/org/kde/desktop/private/TextFieldContextMenu.qml"
qt.qml.typeresolution.cycle: Cyclic dependency detected between "qrc:/qt/qml/org/kde/desktop/private/MobileTextActionsToolBar.qml" and "qrc:/qt/qml/org/kde/desktop/private/TextFieldContextMenu.qml"
qt.qml.typeresolution.cycle: Cyclic dependency detected between "qrc:/qt/qml/org/kde/desktop/private/GlobalSonnetSettings.qml" and "qrc:/qt/qml/org/kde/desktop/MenuItem.qml"
qt.qml.typeresolution.cycle: Cyclic dependency detected between "qrc:/qt/qml/org/kde/desktop/private/MobileTextActionsToolBar.qml" and "qrc:/qt/qml/org/kde/desktop/MenuItem.qml"
qt.qml.typeresolution.cycle: Cyclic dependency detected between "qrc:/qt/qml/org/kde/desktop/private/TextFieldContextMenu.qml" and "qrc:/qt/qml/org/kde/desktop/MenuItem.qml"
qt.qml.typeresolution.cycle: Cyclic dependency detected between "qrc:/qt/qml/org/kde/desktop/private/GlobalSonnetSettings.qml" and "qrc:/qt/qml/org/kde/desktop/private/CheckIndicator.qml"
qt.qml.typeresolution.cycle: Cyclic dependency detected between "qrc:/qt/qml/org/kde/desktop/private/MobileTextActionsToolBar.qml" and "qrc:/qt/qml/org/kde/desktop/private/CheckIndicator.qml"
qt.qml.typeresolution.cycle: Cyclic dependency detected between "qrc:/qt/qml/org/kde/desktop/private/TextFieldContextMenu.qml" and "qrc:/qt/qml/org/kde/desktop/private/CheckIndicator.qml"
qt.qml.typeresolution.cycle: Cyclic dependency detected between "qrc:/qt/qml/org/kde/desktop/private/GlobalSonnetSettings.qml" and "qrc:/qt/qml/org/kde/desktop/private/MobileTextActionsToolBar.qml"
qt.qml.typeresolution.cycle: Cyclic dependency detected between "qrc:/qt/qml/org/kde/desktop/private/GlobalSonnetSettings.qml" and "qrc:/qt/qml/org/kde/desktop/private/TextFieldContextMenu.qml"
qt.qml.typeresolution.cycle: Cyclic dependency detected between "qrc:/qt/qml/org/kde/desktop/private/MobileTextActionsToolBar.qml" and "qrc:/qt/qml/org/kde/desktop/private/TextFieldContextMenu.qml"
qt.qml.typeresolution.cycle: Cyclic dependency detected between "qrc:/qt/qml/org/kde/desktop/private/GlobalSonnetSettings.qml" and "qrc:/qt/qml/org/kde/desktop/MenuItem.qml"
qt.qml.typeresolution.cycle: Cyclic dependency detected between "qrc:/qt/qml/org/kde/desktop/private/MobileTextActionsToolBar.qml" and "qrc:/qt/qml/org/kde/desktop/MenuItem.qml"
qt.qml.typeresolution.cycle: Cyclic dependency detected between "qrc:/qt/qml/org/kde/desktop/private/TextFieldContextMenu.qml" and "qrc:/qt/qml/org/kde/desktop/MenuItem.qml"
qt.qml.typeresolution.cycle: Cyclic dependency detected between "qrc:/qt/qml/org/kde/desktop/private/GlobalSonnetSettings.qml" and "qrc:/qt/qml/org/kde/desktop/private/CheckIndicator.qml"
qt.qml.typeresolution.cycle: Cyclic dependency detected between "qrc:/qt/qml/org/kde/desktop/private/MobileTextActionsToolBar.qml" and "qrc:/qt/qml/org/kde/desktop/private/CheckIndicator.qml"
qt.qml.typeresolution.cycle: Cyclic dependency detected between "qrc:/qt/qml/org/kde/desktop/private/TextFieldContextMenu.qml" and "qrc:/qt/qml/org/kde/desktop/private/CheckIndicator.qml"
kf.kcmutils: Error loading QML file qrc:/kcm/kcm_landingpage/main.qml
kf.kcmutils: "    "qrc:/kcm/kcm_landingpage/main.qml:18:1: Type KCM.SimpleKCM unavailable
    KCM.SimpleKCM {
    ^
kf.kcmutils: "    "qrc:/qt/qml/org/kde/kcmutils/SimpleKCM.qml:33:1: Type Kirigami.ScrollablePage unavailable
    Kirigami.ScrollablePage {
    ^
kf.kcmutils: "    "file:///usr/lib/qt6/qml/org/kde/kirigami/ScrollablePage.qml:68:1: Type Kirigami.Page unavailable
    Kirigami.Page {
    ^
kf.kcmutils: "    "file:///usr/lib/qt6/qml/org/kde/kirigami/Page.qml:237:13: Type Kirigami.PageRow unavailable
                property Kirigami.PageRow row
                ^
kf.kcmutils: "    "file:///usr/lib/qt6/qml/org/kde/kirigami/PageRow.qml:180:5: Type OverlayDrawer unavailable
        property OverlayDrawer leftSidebar
        ^
kf.kcmutils: "    "file:///usr/lib/qt6/qml/org/kde/kirigami/OverlayDrawer.qml:22:1: Type KT.OverlayDrawer unavailable
    KT.OverlayDrawer {
    ^
kf.kcmutils: "    "file:///usr/lib/qt6/qml/org/kde/kirigami/templates/OverlayDrawer.qml:120:56: Cannot assign object of type "KTP.IconPropertiesGroup" to property of type "IconPropertiesGroup_QMLTYPE_60*" as the former is neither the same as the latter nor a sub-class of it.
        property KTP.IconPropertiesGroup handleClosedIcon: KTP.IconPropertiesGroup {
                                                           ^
adam@athena ~❯ systemsettings
KQuickPadding is both anonymous and named
qt.qml.typeresolution.cycle: Cyclic dependency detected between "qrc:/qt/qml/org/kde/desktop/private/GlobalSonnetSettings.qml" and "qrc:/qt/qml/org/kde/desktop/private/MobileTextActionsToolBar.qml"
qt.qml.typeresolution.cycle: Cyclic dependency detected between "qrc:/qt/qml/org/kde/desktop/private/GlobalSonnetSettings.qml" and "qrc:/qt/qml/org/kde/desktop/private/TextFieldContextMenu.qml"
qt.qml.typeresolution.cycle: Cyclic dependency detected between "qrc:/qt/qml/org/kde/desktop/private/MobileTextActionsToolBar.qml" and "qrc:/qt/qml/org/kde/desktop/private/TextFieldContextMenu.qml"
qt.qml.typeresolution.cycle: Cyclic dependency detected between "qrc:/qt/qml/org/kde/desktop/private/GlobalSonnetSettings.qml" and "qrc:/qt/qml/org/kde/desktop/MenuItem.qml"
qt.qml.typeresolution.cycle: Cyclic dependency detected between "qrc:/qt/qml/org/kde/desktop/private/MobileTextActionsToolBar.qml" and "qrc:/qt/qml/org/kde/desktop/MenuItem.qml"
qt.qml.typeresolution.cycle: Cyclic dependency detected between "qrc:/qt/qml/org/kde/desktop/private/TextFieldContextMenu.qml" and "qrc:/qt/qml/org/kde/desktop/MenuItem.qml"
qt.qml.typeresolution.cycle: Cyclic dependency detected between "qrc:/qt/qml/org/kde/desktop/private/GlobalSonnetSettings.qml" and "qrc:/qt/qml/org/kde/desktop/private/CheckIndicator.qml"
qt.qml.typeresolution.cycle: Cyclic dependency detected between "qrc:/qt/qml/org/kde/desktop/private/MobileTextActionsToolBar.qml" and "qrc:/qt/qml/org/kde/desktop/private/CheckIndicator.qml"
qt.qml.typeresolution.cycle: Cyclic dependency detected between "qrc:/qt/qml/org/kde/desktop/private/TextFieldContextMenu.qml" and "qrc:/qt/qml/org/kde/desktop/private/CheckIndicator.qml"
qt.qml.typeresolution.cycle: Cyclic dependency detected between "qrc:/qt/qml/org/kde/desktop/private/GlobalSonnetSettings.qml" and "qrc:/qt/qml/org/kde/desktop/private/MobileTextActionsToolBar.qml"
qt.qml.typeresolution.cycle: Cyclic dependency detected between "qrc:/qt/qml/org/kde/desktop/private/GlobalSonnetSettings.qml" and "qrc:/qt/qml/org/kde/desktop/private/TextFieldContextMenu.qml"
qt.qml.typeresolution.cycle: Cyclic dependency detected between "qrc:/qt/qml/org/kde/desktop/private/MobileTextActionsToolBar.qml" and "qrc:/qt/qml/org/kde/desktop/private/TextFieldContextMenu.qml"
qt.qml.typeresolution.cycle: Cyclic dependency detected between "qrc:/qt/qml/org/kde/desktop/private/GlobalSonnetSettings.qml" and "qrc:/qt/qml/org/kde/desktop/MenuItem.qml"
qt.qml.typeresolution.cycle: Cyclic dependency detected between "qrc:/qt/qml/org/kde/desktop/private/MobileTextActionsToolBar.qml" and "qrc:/qt/qml/org/kde/desktop/MenuItem.qml"
qt.qml.typeresolution.cycle: Cyclic dependency detected between "qrc:/qt/qml/org/kde/desktop/private/TextFieldContextMenu.qml" and "qrc:/qt/qml/org/kde/desktop/MenuItem.qml"
qt.qml.typeresolution.cycle: Cyclic dependency detected between "qrc:/qt/qml/org/kde/desktop/private/GlobalSonnetSettings.qml" and "qrc:/qt/qml/org/kde/desktop/private/CheckIndicator.qml"
qt.qml.typeresolution.cycle: Cyclic dependency detected between "qrc:/qt/qml/org/kde/desktop/private/MobileTextActionsToolBar.qml" and "qrc:/qt/qml/org/kde/desktop/private/CheckIndicator.qml"
qt.qml.typeresolution.cycle: Cyclic dependency detected between "qrc:/qt/qml/org/kde/desktop/private/TextFieldContextMenu.qml" and "qrc:/qt/qml/org/kde/desktop/private/CheckIndicator.qml"
kf.kcmutils: Error loading QML file qrc:/kcm/kcm_landingpage/main.qml
kf.kcmutils: "    "qrc:/kcm/kcm_landingpage/main.qml:18:1: Type KCM.SimpleKCM unavailable
    KCM.SimpleKCM {
    ^
kf.kcmutils: "    "qrc:/qt/qml/org/kde/kcmutils/SimpleKCM.qml:33:1: Type Kirigami.ScrollablePage unavailable
    Kirigami.ScrollablePage {
    ^
kf.kcmutils: "    "file:///usr/lib/qt6/qml/org/kde/kirigami/ScrollablePage.qml:68:1: Type Kirigami.Page unavailable
    Kirigami.Page {
    ^
kf.kcmutils: "    "file:///usr/lib/qt6/qml/org/kde/kirigami/Page.qml:237:13: Type Kirigami.PageRow unavailable
                property Kirigami.PageRow row
                ^
kf.kcmutils: "    "file:///usr/lib/qt6/qml/org/kde/kirigami/PageRow.qml:180:5: Type OverlayDrawer unavailable
        property OverlayDrawer leftSidebar
        ^
kf.kcmutils: "    "file:///usr/lib/qt6/qml/org/kde/kirigami/OverlayDrawer.qml:22:1: Type KT.OverlayDrawer unavailable
    KT.OverlayDrawer {
    ^
kf.kcmutils: "    "file:///usr/lib/qt6/qml/org/kde/kirigami/templates/OverlayDrawer.qml:120:56: Cannot assign object of type "KTP.IconPropertiesGroup" to property of type "IconPropertiesGroup_QMLTYPE_52*" as the former is neither the same as the latter nor a sub-class of it.
        property KTP.IconPropertiesGroup handleClosedIcon: KTP.IconPropertiesGroup {
                                                           ^
qt.qml.typeresolution.cycle: Cyclic dependency detected between "qrc:/qt/qml/org/kde/desktop/private/GlobalSonnetSettings.qml" and "qrc:/qt/qml/org/kde/desktop/private/MobileTextActionsToolBar.qml"
qt.qml.typeresolution.cycle: Cyclic dependency detected between "qrc:/qt/qml/org/kde/desktop/private/GlobalSonnetSettings.qml" and "qrc:/qt/qml/org/kde/desktop/private/TextFieldContextMenu.qml"
qt.qml.typeresolution.cycle: Cyclic dependency detected between "qrc:/qt/qml/org/kde/desktop/private/MobileTextActionsToolBar.qml" and "qrc:/qt/qml/org/kde/desktop/private/TextFieldContextMenu.qml"
qt.qml.typeresolution.cycle: Cyclic dependency detected between "qrc:/qt/qml/org/kde/desktop/private/GlobalSonnetSettings.qml" and "qrc:/qt/qml/org/kde/desktop/MenuItem.qml"
qt.qml.typeresolution.cycle: Cyclic dependency detected between "qrc:/qt/qml/org/kde/desktop/private/MobileTextActionsToolBar.qml" and "qrc:/qt/qml/org/kde/desktop/MenuItem.qml"
qt.qml.typeresolution.cycle: Cyclic dependency detected between "qrc:/qt/qml/org/kde/desktop/private/TextFieldContextMenu.qml" and "qrc:/qt/qml/org/kde/desktop/MenuItem.qml"
qt.qml.typeresolution.cycle: Cyclic dependency detected between "qrc:/qt/qml/org/kde/desktop/private/GlobalSonnetSettings.qml" and "qrc:/qt/qml/org/kde/desktop/private/CheckIndicator.qml"
qt.qml.typeresolution.cycle: Cyclic dependency detected between "qrc:/qt/qml/org/kde/desktop/private/MobileTextActionsToolBar.qml" and "qrc:/qt/qml/org/kde/desktop/private/CheckIndicator.qml"
qt.qml.typeresolution.cycle: Cyclic dependency detected between "qrc:/qt/qml/org/kde/desktop/private/TextFieldContextMenu.qml" and "qrc:/qt/qml/org/kde/desktop/private/CheckIndicator.qml"
kf.kcmutils: Error loading QML file qrc:/kcm/kcm_screenlocker/main.qml
kf.kcmutils: "    "qrc:/kcm/kcm_screenlocker/main.qml:15:1: Type KCM.SimpleKCM unavailable
    KCM.SimpleKCM {
    ^
kf.kcmutils: "    "qrc:/qt/qml/org/kde/kcmutils/SimpleKCM.qml:33:1: Type Kirigami.ScrollablePage unavailable
    Kirigami.ScrollablePage {
    ^
kf.kcmutils: "    "file:///usr/lib/qt6/qml/org/kde/kirigami/ScrollablePage.qml:68:1: Type Kirigami.Page unavailable
    Kirigami.Page {
    ^
kf.kcmutils: "    "file:///usr/lib/qt6/qml/org/kde/kirigami/Page.qml:237:13: Type Kirigami.PageRow unavailable
                property Kirigami.PageRow row
                ^
kf.kcmutils: "    "file:///usr/lib/qt6/qml/org/kde/kirigami/PageRow.qml:180:5: Type OverlayDrawer unavailable
        property OverlayDrawer leftSidebar
        ^
kf.kcmutils: "    "file:///usr/lib/qt6/qml/org/kde/kirigami/OverlayDrawer.qml:22:1: Type KT.OverlayDrawer unavailable
    KT.OverlayDrawer {
    ^
kf.kcmutils: "    "file:///usr/lib/qt6/qml/org/kde/kirigami/templates/OverlayDrawer.qml:120:56: Cannot assign object of type "KTP.IconPropertiesGroup" to property of type "IconPropertiesGroup_QMLTYPE_52*" as the former is neither the same as the latter nor a sub-class of it.
        property KTP.IconPropertiesGroup handleClosedIcon: KTP.IconPropertiesGroup {

EXPECTED RESULT
works

SOFTWARE/OS VERSIONS
Operating System: Arch Linux 
KDE Plasma Version: 5.91.0
KDE Frameworks Version: 5.247.0
Qt Version: 6.7.0
Kernel Version: 6.6.7-arch1-1 (64-bit)
Graphics Platform: Wayland

ADDITIONAL INFORMATION

Loading a KCM manually with kcmshell works but some of them seem kind of flaky.

This seems likely to be breakage related to Qt 6.7, and as such might be related to Bug 478815.