Hi Fiona,
Welcome to Apple Discussions.
It's possibly related to Privoxy. Temporarily disabling it will solve the problem. Adding '.apple.com' to Privoxy's fragile sites list (under site-specific rules in the default.action file) will fix it permanently.
If you're not running Privoxy, it may be related to other proxies, firewalls or such that you may be running.
Regards,
Peter.