I had something similar a couple of weeks ago. Two different error messages, not sure which ones though (-400, maybe?).
I had to delete some cookies from my machine to make it work. I went to Safari > Preferences > Security (show cookies). There were a few specific ones causing me problems; these I believe: s_vi_hyy, s_vi_hyy, s_vi_ixx (and I think they were connected to apple.com).
Once I deleted the offending cookies, all was well.
Hope this helps. If not, continue to look into the cookie issue and you should find what works. 🙂
Cheers,
Josh