Thanks for searching Rob, the D810 is supported by DxO OpticsPro which somehow did not make the D810A files readable. Darktable and Gimp seem to be having issues too ... so yeah, one way or an other PixInsight does not have such problems, and if a dev reads this I'd appreciate some insight (no pun intended).
P.S. : I remember Apple's Preview didn't read the D810A raw files either, but at some point an update allowed it