I'm seeing this issue too and have confirmed it with wget. We're seeing autoinstall.plesk.com at 72.21.81.253. Some files work where others don't:
[root@x in]# wget https://autoinstall.plesk.com/sso.inf3
--2012-11-16 18:02:53-- https://autoinstall.plesk.com/sso.inf3
Resolving...