Ubuntu 18.04 server installation gets stuck at 66% while running'update-grub'

Not a solution but a couple of Workarounds. Apparently, that's a bug in os-prober . I personally tried the second one and it works! To quote from the link:

Workaround 1: (proaction)

When you are reaching
the “Install the GRUB boot loader to the master boot record?” prompt, (in my case, no such prompt appeared but i figured out timing of the grub-install) switch to a console (ctrl+alt+[f2-f6]), and remove this file:

rm /target/etc/grub.d/30_os-prober

This will prevent update-grub from running os-prober, which should avoid running into this issue. Of course, other operating systems won't be listed, but at least that should prevent the installation process from getting entirely stuck. I've tested this successfully in a VM with guided (unencrypted) LVM, and standard plus ssh tasks (which is how I initially reproduced your issue).

Workaround 2: (reaction)

Otherwise, once the process is stuck, locate the process identifier (PID) on the first column of the ps output:

ps | grep 'dmsetup create'

then kill this dmsetup process. With your output above, that'd be:

kill 19676

(Tested successfully in a VM with the same setup/choices as above.)

KiBi