Permission denied error

I have a HTML page (e.g. index.html) on my computer. This page contains 3
IFRAMEs where each iframe loads a web page from a different web site:

1) a page is loaded from "http://xxx.institution.code"
2) a page is loaded from "http://local_1.institution.town.code"
3) a page is loaded from "http://local_2.institution.town.code/somescript"

One of these IFRAMEs (the frist one in the list above) loads an HTML document
which in turn loads a javascript file. At a certain moment the code in the
javascript file executes :

if( window.location.search == "" )
{
... do something ...
}

and a "Permission denied" error occurs.

1) On my computer located in "institution" which does not use any HTTP proxy
the error does not occur.

2) On another computer located in institution the error does not occur
if the HTTP proxy is set up manualy.

3) On the same another computer the error occurs if the HTTP proxy is
configured with group policies through an automatic configuration script.

Can somebody explain my why this behaviour ?

Another thing which is not clear for me is that I could understand the error
in the page "http://xxx.institution.code" if it were trying to read the
location of a window corresponding to another frame, but since it reads the
location of its own window I don't see anything wrong in this.

Thx, Cristi