If the M.2_1 is occupied, the SATA 5 and 6 do not work. If the M.2_2 is occupied, the SATA 3 and 4 do not work.
Since I did not read this when I bought the notebook and therefore did not know it, I assumed the motherboard was defective when I tested it. However, the ASUS support made me aware of the "concept" and this was clarified.
Hopefully saves one or the other long troubleshooting ;)