There seem to be some invalid plasmoids? Or an unfinished update? The error messages like “PlasmaCore.IconItem is not a type” look like a Plasma5 version of some plasmoid as that should by now be “Kirigami.Icon” (see develop.kde.org/docs/plasma/widget/porting_kf6/ )
I’m not sure which plasmoid is actually giving the error message, so it might be a third-party one, that’s to blame?