[squid-users] NTLM auth to remote server fails through squid

We're trying to connect to a remote server that
requires authentication. This works fine when
we place the browser client on the Internet, but
when we place the browser client behind squid the
authentication popup just returns without accepting
the login.

I have tried Squid 3.1.19 and 3.2.0.18. Browsers are
IE 9 and Firefox 13.

Here is an extract of the HTTP Server reply:
Connection: Keep-Alive
Server: Microsoft-IIS/6.0
WWW-Authenticate: Negotiate
WWW-Authenticate: NTLM

The squid configuration in the test server is default,
except that I have added visible_hostname and changed
http_port to 80.

What could be the reason for this auth failure?
What debug values should I use?

NB: This is not about authenticating to the proxy server,
we allow proxy connections from inside without authentication.
The question is about authenticating to an external server
that is out of our control.

Thanks!

-- 
Peter Olsson                    pol_at_leissner.se