Did you make an account or change your password recently?
Someone just emailed me yesterday reporting that after changing their Apple ID password, they were suddenly unable to log into iCloud on Mavericks anymore (or on iOS 6) where it had previously worked. I'm wondering if Apple implemented some sort of new password-hashing system which Mavericks is incompatible with.
Does anyone have a "burner" iCloud account they don't mind losing access to on Mavericks to see if they can reproduce this?
@iPodNano3 this might be somewhere your epic CharlesProxy skills could really come in handy!