Thank you a lot to all of you. This explains very well the problem, and it seems that @Curefab suggestion is worth an attempt: using the keyword, a lot of material and suggestions are available, here just a couple of examples:
Just a word about the OEM consideration: this is a low budget project, so, not so much money and in counterparts a lot of time to spend, also in learning through errors. I also would have a couple of stories about OEMs, but… ok, not here.
about the RTX8000 and ML350g10: the machine does support a maximum of two of them (with an “HPE” added in the beginning of the name…).
Interesting, and I cannot be sure, but probably this was not clearly stated in the beginning. It is specified in an update of the docs, and probably some “4 GPUS in one machine” accidents happened, maybe even on OEMs prepared machines:
https://support.hpe.com/hpesc/public/docDisplay?docId=a00101748en_us&docLocale=en_US
Thank you, really, to all of you for your help.
Enrico