I think I have the same problem.
I did a fresh install of 20.04 on a Lenovo Carbon X1 6th gen. Previously
it was working fine under 18.04.
The 20.04 install ran perfectly, and at first the system booted up and
ran normally.
Shortly afterward I got a notice about a Thunderbolt firmware update
from Lenovo. After I installed the firmware update, Plymouth started
hanging.
The bug occurs when an external monitor is plugged in.
Removing splash option in grub fixes the problem.
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https://bugs.launchpad.net/bugs/1871641
Title:
Ubuntu never finishes booting: A start job is running for Hold until
boot process finishes up (3min 7s / no limit)
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