It could be that the IP address range is, for some obscure reason, blocked.
However you would not get the "page can't be displayed" page if that were the case , so that don't make much sense...
I came across an article dealing with a similar problem where it was suggested to lower the MTU size, this is not something that you could do yourself..
MTU is the Maximun Transmission Unit , that will be allowed through by a router , it's like saying the maximun size letter you will accept for a given letter box, if the letter is to big then it won't fit in the letterbox :-)
It could be that the server this site is hosted on has a MTU that is smaller than the MTU your server is sending out . IE your server is sending a bigger letter than the "letterbox" of this server, this server won't know what hit it and just reject it .
It's a possibility , but a slim one !
You should not get the page if the site is blocked by any type of site blocker program , so i doubt that either..
Odd problem, will have to think about that :-)