i had this problem 2 days ago. i had old vps 2 days before i realise that i didint have seen those avast update pop ups in some time.
also nothing had changed two days in C:\Program Files\AVAST Software\Avast\defs
it just keep saying "unknown error" when i tried to manually update it.
i managed to fix it by:
1.downloaded manual update and save it to desktop
(manual update in normal mode worked, but it didint change those vps numbers you can see in update screen or when you put mouse over avast icon in low right corner, and manual update still failed at "unknown error")
2.repaired avast!, wait 5 min and close the computer
3.launch computer is safemode
4.run manual update and rebooted to normal mode
5.now when i tried to update, it say that virus definitions are already up to date
6.couple hours later i saw the update pop up.
i dont know what fixed it, but it is working now.
my avast versio is 2014.9.0.2013 and im using Avast! Internet Security
my avast is using hxxp://x9942723.ivps9x.u.avast.com/ivps9x (i changed tt to xx making it unclickable)
was there any problems in that server couple days ago?