Hi Paddy,
The MOK screen was behaving as expected (it only appears on the first boot after installing Ubuntu). Yes, its a new machine (Lenovo x380), and I'm installing Ubuntu 18.04.1.
I did some more exploring with grub and (to cut a long story short) discovered this bug, which clearly explains that "Ubuntu cannot boot from an encrypted volume with Secure Boot enabled" because the required grub modules (luks and cryptodisk) are not included in the distributed signed grub binary, and, for security reasons, grub will not load them subsequently (so the "insmod" commands fail - though they return a success exit status nevertheless).
So, I disabled UEFI Secure Boot, and found that I now get
Attempting to decrypt master key...
Enter passphrase for hd0,gpt5 (1ee8a2ca...):
All well and good. Except that after I enter the passphrase, nothing happens for 60 seconds. Then I get the Ubuntu startup screen, and then after another 65 seconds, I get the Busybox ... (initramfs) screen. "Exit" then tells me: "ALERT! /dev/mapper/system-root does not exist.", and I'm taken back to the initramfs prompt again. If I then do "ls /dev/mapper", I find that the only thing listed is "control". So, it seems that the boot process hasnt completed properly and found the (unlocked?) partitions with Ubuntu on.
I havent yet started searching for solutions to this, but I wondered if you had come across this yourself?
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