That must be it. I'd bet there must be an error message hanging on the real desktop. Do you have physical access to the server (and does it have a monitor attached)? If so, please check it out - it's pretty likely you'll see something there.
Or if you don't, try the following (still in front of the physical machine): kill the avast.setup process, and again, invoke manually the update. You should see a message on the desktop (possibly an error message, given that it was blocking the updates previously).
Thanks (and sorry for that)
Vlk