This tip is derived from the thread started by
by roto. Corrected a couple typos and after the quote added some commentary:
After searching around for a longer time how to fix this problem in Software-Update, I finally found out how to manage this:
1. Delete PLISTs
[/ indicates the path from the boot hard drive to folders inside. Use Go to Folder in the Go menu to navigate to that folder
replacing <my username> with the login user trying to access the store, the file in question is after the last slash. Quitting from the Mac App Store is recommended before you delete these files, by going to the Mac App Store menu and selecting Quit.]
2. In Applications -> Utilities -> Terminal type or copy/paste:
sudo killall -HUP mDNSResponder
[Hit enter/return key when done typing the above. Enter your administrative password when requested followed by return/enter key.]
Mac OS X 10.6.6 to 10.6.8 users should use instead:
sudo dscacheutil -flushcache
Source: OS X: How to reset the DNS cache
1. Open System Preferences - Network
2. Highlight active connection
3. Click Advanced.
5. Renew DHCP Lease
Step 2, mDNSResponder issues should not happen unless:
1. The internet service provider is not supporting your IPv6 or IPv4 configuration.
2. The internet service provider has latency issues, such as satellite connections.
3. The internet service provider has a proxy/firewall configuration getting in the way. Often this setup is common with schools and corporate networks.
4. The DNS at the internet service provider is corrupted. To isolate this, change the DNS to an http://www.opendns.org/ one.
Also generally speaking sudo commands should not be issued, or moving any files after a sudo command without ensuring file sharing is first turned off. The good portion of this tip is the reboot which clears the login for sudo, which can leave your machine vulnerable until the reboot has been executed.
Software such as Little Snitch, and software firewalls may also interfere with the connection, as well as peer2peer software.