I've just installed U-M 18.04 on an HP Envy laptop and done an update. On shutdown it hangs with error message:
[...] watchdog: BUG: soft lockup - CPU#0 stuck for 22s! [kworker/0:2:62] which repeats every second. After a bit CPU#6 joins in referring to [plymouthd:2976]
I have to hold down the power button to stop it. On starting it again a U_M popup tell me there was a "System program problem detected" but no way of seeing what it was.