https://bugs.launchpad.net/ubuntu/+source/linux-signed/+bug/1876593Schonmal ein Bios-Update gemacht?
Frustratingly, this seems to have now resolved itself, so I can't test the patch. All I did was leave the machine off for a few hours and turn it back on - then it was working again.I now can't reproduce the problem, with or without the Lan Boot ROM option. The PHY ID now reports as 0x001cc912.I'll come back to it later and see if it breaks again.