It seems that Cactus applies the CCTK_CENTERING_GF macro not just to grid functions, but also to grid scalars. This fails at run time since CarpetX notices that the GF3D2 class doesn’t work for grid scalars.

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Ticket URL: https://bitbucket.org/einsteintoolkit/tickets/issues/2600/cctk_centering_gf-is-applied-to-grid