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:///opt/kde6/lib/x86_64-linux-gnu/qml/org/kde/kirigami/ScrollablePage.qml:67:1: Type Kirigami.Page unavailable
    Kirigami.Page {
    ^
kf.kcmutils:     file:///opt/kde6/lib/x86_64-linux-gnu/qml/org/kde/kirigami/Page.qml:237:13: Type Kirigami.PageRow unavailable
                property Kirigami.PageRow row
                ^
kf.kcmutils:     file:///opt/kde6/lib/x86_64-linux-gnu/qml/org/kde/kirigami/PageRow.qml:180:5: Type OverlayDrawer unavailable
        property OverlayDrawer leftSidebar
        ^
kf.kcmutils:     file:///opt/kde6/lib/x86_64-linux-gnu/qml/org/kde/kirigami/OverlayDrawer.qml:22:1: Type KT.OverlayDrawer unavailable
    KT.OverlayDrawer {
    ^
kf.kcmutils:     file:///opt/kde6/lib/x86_64-linux-gnu/qml/org/kde/kirigami/templates/OverlayDrawer.qml:120:56: Cannot assign object of type "KTP.IconPropertiesGroup" to property of type "IconPropertiesGroup_QMLTYPE_91*" as the former is neither the same as the latter nor a sub-class of it.
        property KTP.IconPropertiesGroup handleClosedIcon: KTP.IconPropertiesGroup {
                                                           ^

So the type given to the property is somehow not the same as the one used for constructing the object. When commented out the next error is:
kf.kcmutils:     file:///opt/kde6/lib/x86_64-linux-gnu/qml/org/kde/kirigami/templates/OverlayDrawer.qml:101:63: Cannot assign object of type "KTP.IconPropertiesGroup" to property of type "IconPropertiesGroup_QMLTYPE_58*" as the former is neither the same as the latter nor a sub-class of it.
        readonly property KTP.IconPropertiesGroup handleOpenIcon: KTP.IconPropertiesGroup

And once that is commented out, things work but we still get these errors in the konsole:
ile:///opt/kde6/lib/x86_64-linux-gnu/qml/org/kde/kirigami/ApplicationItem.qml:95: TypeError: Cannot call method 'forceActiveFocus' of null
file:///opt/kde6/lib/x86_64-linux-gnu/qml/org/kde/kirigami/templates/AbstractApplicationHeader.qml:39:5: Unable to assign PageRow_QMLTYPE_213_QML_250 to PageRow_QMLTYPE_9
file:///opt/kde6/lib/x86_64-linux-gnu/qml/org/kde/kirigami/private/globaltoolbar/PageRowGlobalToolBarUI.qml:138:13: Unable to assign PageRow_QMLTYPE_213_QML_250 to PageRow_QMLTYPE_9
file:///opt/kde6/lib/x86_64-linux-gnu/qml/org/kde/kirigami/Page.qml:203: Error: Cannot assign QObject* to PageRow_QMLTYPE_9*