Last Night, on starting a computer running Avast (Free). It remained on the Blank Black part of the boot for several minutes, C.A.D to The task manager revealed ‘Avast emergency updater’ was running. eventually the boot completed.
I assume that Avast was doing an update during boot up, but with no warning and no explanation afterwards I have 9.0.2018 but that has been current for a couple of weeks.
Then a second computer did exactly the same thing (very long blank screen during boot).
Can anyone explain what is going on?

Could be the server was busy or was having some problems that caused the slowdown.
Keep an eye on it for the next couple of days to see if it happens again.