I'm trying to upgrade a working Leap 42.3 to Leap 15.0 with the
"openSUSE-Leap-15.0-NET-x86_64267" ISO image. Network connection is via wired
ethernet.
Previous upgrades, for example from 42.2 > 42.3, worked flawlessly.

Boot parameter: ipv4only=1

All I can see is:

Loading basic drivers... ok
...
Realtek RTL8111/8168/8411 PCI Express Gigabit Ethernet
  drivers: r8169*
eth0: network config created
Sending DHCP4 request to eth0...
DHCP configuration failed.

Omitting the "ipv4only=1" boot parameter leads to the same result (the router
is IPv4 only).