I checked all the traces on the CHAFIO board but didn't find any broken one.
So I removed all the C ROM chips (8 of them, 42 pins each!):
I dumped them all and they were all identical to the MAME romset v1.
I finally transplanted them (and the M1 ROM) on a KOF99 board as I was pretty sure the CMC custom chip died (pretty common unfortunately).