I want to add that adding:
using namespace CesiumGltf;
Just after
END_DEFINE_SPEC(...)
in the files CesiumPropertyTextureProperty.spec.cpp and CesiumPropertyTexture.spec.cpp has solved the issue and Cesium builds successfully. Thank you for leading us to the solution.
We still don’t understand why an older project with the same cesium for UE version and identical build settings does not need the modifications though.