error 500

I'm having a problem getting to sites where i can check a database for books. i can get to the main page, but when i try to look for a book, specifically on www.abebooks.com, i get this error:

Explanation: There is a problem with the page you are trying to reach and it cannot be displayed.

Try the following:

* Refresh page: Search for the page again by clicking the Refresh button. The timeout may have occurred due to Internet congestion.
* Check spelling: Check that you typed the Web page address correctly. The address may have been mistyped.
* Access from a link: If there is a link to the page you are looking for, try accessing the page from that link.

If you are still not able to view the requested page, try contacting your administrator or Helpdesk.

Technical Information (for support personnel)

* Error Code: 500 Internal Server Error. The request is not supported. (50)
* IP Address: xx.xx.xx.xx
* Date: 8/13/2008 1:57:01 PM
* Server: firewall server
* Source: proxy

anyone know why this is happening? is it blocked somehow on my firewall? i use an ISA server as a firewall.