Possibility #1
Quick look at the source code for that site shows that they make use of the referer information - this is used by some sites to prevent pages being embedded in other sites. Norton Internet Security blocks that kind of thing - way round on
Symantec site (the document mentions v. 2002 but the 'problem' occurs with other versions too)
You may need to add the site to your allowed sites even if you are using a bookmark - I had the same problem with an aviation pic site.
Try that - see what happens - can't do any harm and it is easily reversible. If it doesn't work I'll have to think about a possiblity #2 ...