Comment 22 for bug 631426

I too ran into this issue. I worked around it by reading comments from other users on this page.

We use a proxy script at our workplace. I have specified the script URL in the "Automatic proxy configuration" setting of [System->Preferences->Network Preferences]. I extracted the HTTP proxy details from the script and set it in the HTTP proxy field of "Manual proxy configuration" setting of [System->Preferences->Network Preferences]. After this change, my upgrade worked fine (actually it is still going on but it has gone past the point where I used to see the problem). I'll restore the network preferences after the upgrade.