I've configured my IIS web site to use a couple of custom error
pages that I've added as html pages in the root of my sps site, 401-
1.htm and 401-2.htm.

The error pages are for errors 401.1 and 401.2. These should show up
when a user doesn't provide her credentials when trying to access
the SPS site. I have a 2 server farm and on one server, the user
will see the custom error page, on the other server, the user gets a
SPS error page that says "Access denied. You do not have permission
to perform this action or access this resource." The IIS and file-
level security settings are identical for the error pages on both
servers. I can't figure out why the servers work differently.

The only other thing I can think to mention is that on the server
where the custom error pages do not appear, the browser attempts to
load default.aspx briefly before displaying the error message (using
reqacc.aspx).

What is the right way to enable custom IIS error pages under SPS?