I think I've tracked down the issue. UM/Qt/qml/UM/SimpleButton.qml references a UI element called "ColorImage". This class exists in Qt, but it's private and was probably changed/removed in Qt6.
However, there's also a ColorImage class in Uranium (Cura's UI toolkit), and I suspect they actually wanted to use that, but forgot to reference the correct type. They actually fixed this upstream: https://github.com/Ultimaker/Uranium/commit/88fd460a63263d66173f1e0127b0f208577d7f4e I'm going to add this patch to the Uranium package.
