update to 15.1, kernel seems to be a deadbeat dud

It was working fine with Leap 15.0 and KDE desktop. I updated to 15.1 using a DVD. So it boots, and gets to the
Leap 15.1 boot screen
then the last thing that appears is
loading init ramdisk

before the screen goes blank. I noticed also that the lights on the ethernet port are not lit up.
It is an old system: i7 P55 chipset, so hardly new unsupported hardware.
Only if I select the old kernel 4.12.14lp150 will it boot up to a functioning GUI desktop.
So I guess I must wait for a new kernel to emerge in coming updates?

Nvidia ?

Could be this: https://bugzilla.opensuse.org/show_bug.cgi?id=1136978

In which case a fix is fairly imminent.

But… if it is indeed the nouveau bug, you should be able to install and boot the “as released” 4.12.14-lp151.27.3 kernel, locking it and not updating to the latest update, until the fixed kernel update shows up.
Or, you can boot to the lp150 kernel that seems to work for you, uninstall the lp151.28 broken kernel and install the 27.3 in its place.