One user asked for User/Pass, others not (Integrated Authentication)

Hi folks

I have an ASP.NET application using Windows Integrated Authentication, where one user cannot log in. He is asked for his Username and Password. No one else has this problem, despite being members of the same groups. I'm not using impersonation...

In my web.config, in the general section, my authorisation is as follows:

        <allow users="*"/> <!-- Allow all users -->


Though I do have one location-specific authorization which denies most users, it is not for the file that this user is trying to access (besides, other users who can log on to the main page are also denied to this location)

What things should I be checking to figure out why this user is denied access but everyone else is not?

Thanks for any help!
...