[squid-users] WWW-Authenticate error

We are migration from ISA to Squid and have following problem:
When i want to connect to one website without squid with IE6 browser i get
an windows authenthication window, however with squid enabled i get
following error:
You do not have permission to view this directory or page using the
credentials that you supplied because your Web browser is sending a
WWW-Authenticate header field that the Web server is not configured to
accept.

Is there a ways to solve this problem.
Thanks

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