Interesting problem!
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 :-)