(I was going to post the journal lines, but sys-whonix journal data gets reset after reboot)
Apparently, ipv6 in sys-net and sys-firewall was enabled.
After explicitly disabling ipv6,
qvm-features sys-firewall ipv6 '' and also for the sys-net,
the eth0 interface in sys-whonix is up.
systemcheck also reports OK.
I think whonix-qubes documentation might consider add this as a troubleshooting step.