Problem accessing specific web site

Hi. I have a MN700, which I use without problem to access
the internet. I do have a problem with a specific web site
(www.amcostarica.com), which I cannot access using the
wireless connection.
I know that the site is OK because I can access it if I
connect directly to the internet, bypassing the MN700.
The problem occured with Windows XP, and still persists
after I upgraded to the new XP SP-2.
I wonder if someone can help me.
Mathias