IPcop: problem with upstream proxy authentication

Hello!
I am running ipcop 1.4.13 and I have enabled the web proxy on the
green interface.
I have configured an Upstream Proxy (ISA server) which requires
authentication (so I've put username and password in the Upstream
Username and Upstream Password fields).
I have configured the proxy settings in the browser on a machine on
the green network, to point to the proxy service of ipcop.

When I try to access the Internet from that machine, the ipcop proxy
is contacted, the request is then passed to the upstream proxy, but I
get an error:
Error Code: 407 Proxy Authentication Required. The ISA Server requires
authorization to fulfill the request. Access to the Web Proxy filter
is denied. (12209)

It seems to me that the authentication credentials are not passed to
the upstream proxy...
Is there something wrong in my configuration? Is there any
compatibility issue in order to use ISA server as an upstream proxy
for Squid 2.5stable14?
Any help or suggestion would be greatly apreciated!
Thanks a lot!
Filippo