I had a similar problem. I can't access iTunes, but my co-worker (using the same network) can. Moving my preferences out of ~/Library/Preferences/com.apple.iTunes* and attempting to set up the account again doesn't help. The Keychain tool says nothing is wrong. It turned out I had two problems:
- I had a proxy setting to use a "proxy.pac" file. It turned out this proxy.pac file basically had "Go Direct" in it. Safari, etc, handled this correctly, and iTunes used to, but it stopped working in a recent upgrade (I think around MacOSX 10.5.3.)
- Turn IPv6 support off from the "Advanced" setting in the Network panel of System Preferences.
It tried with both of these individually and it didn't help, I had to do
both to make iTunes work again.
The reference to the proxy.pac file I'd had there for a year and a half. We
do have functioning IPv6 in the office, and our ISP has other IPv6 customers, but external IPv6 connectivity is a bit spotty to say the least. I suspect the problem only applies to those who have local but not global IPv6 connectivity.
One wonders if the person who said it occurred after a network upgrade, was due to their ISP now supported IPv6 and triggering this problem?