OK,
some news here after full rebuild of server.
MOBO failure was an extra, still VMs do not reboot.
well, not exactly...
we have installed some windows core, some ubuntu bare server, some windows full gui, some ubuntu server with full MATE desktop.
now the "barebone" VMs do not reboot, they just shutdown.
The full bloated VMs restart nicely!
I think this is something to do with the speed of the reboot cycle... but even my reseller tech ignores what to do... HPE is finally thinking to pass me to L2 support.
project is delayed at least of 1 week wrt deadlines (and I've kept 1 extra week at the time, just in case)