Custom ErrorDocument doesn't display correctly

Question

I have created a custom ErrorDocument 401 to use in
.htaccess. After the user cancels out of the login screen in
Firefox or Opera, they are redirected to the authreqd.html file for
the custom error message. However, when that page is diplayed in
Firefox or Opera, none of the background images load. The Alt text,
when available, is shown instead. Why would the images not load for
this file when they work correctly for all other links.
Interestingly, when you type in the URL and go directly to that
page, it does display correctly.
P.S. Safari doesn't load the custom error page at all. An
incorrect username/password returns the login screen repeatedly,
and cancelling returns you to the page you were previously on.
Here is the custom error
file

Answer 1

There are tonnes of possibilities and many websites with suggestions. I tried all sorts of these option (clear cache, clear cookies, anti-virus changes, page style options etc) with no success until eventually I accidentally changed my character encoding autodetect (firefox 6) from "off" to "universal" and everything is fine.