(I found the solution to this problem while writing up my question , but I'm posting in case it can help someone else)
I downloaded Windows 8 RTM version from MSDN and I'm having the following issues following the upgrade from Windows 7 Pro:
1) Online activation (from the command line or from the UI) does not work and returns error 0x80072EFD which corresponds to "A connection with the server could not be established"
2) The store cannot be accessed, and simply displays "We weren't able to connect to the store"
3) Mail app says "We're having trouble signing you in. Try again in a few minutes."
4) Windows Update crashes while scanning for updates. The event viewer also displays EventID 1001 with Event Name "WindowsUpdateFailure2" / Response "Not available" periodically.
I can confirm that my internet connection is working, both in classic and 'modern UI' Windows (Weather and News apps work, at least). To resolve the issue, I tried the following:
A) Reset firewall settings
B) Uninstall any network-related 3rd party apps (such as the FortiSSL VPN client)
C) Un-join my computer from the corporate domain
D) Redo the upgrade process from scratch
E) Tried to set the DisableWindowsUpdateAccess reg setting to 0 as outlined in this article: http://johndandison.com/blog/post/2012/04/05/Windows-8-Metro-Apps-Store-Windows-Update-WSUS.aspx
None of the above solutions worked. I am not behind a proxy or any special firewall, so I don't think it explains the issues that I'm having. Since I'm running under a VM, I decided to take a snapshot of my VM and try the 'Refresh my PC' option, which
resolved the problem. However I lost all my previously installed Windows apps.
Update: I found a solution to my problem. All I had to do was run netsh winhttp reset proxy in the command line.
