security prob when accessing restricted webpage

Hi,

i have a IIS 6 on Windows 2003.

On one of the websites i host, they have one ASP they use for entering data.
They wanted to secure that one page. So i made a user with a password, then
in the Windows Explorer on the security settings of that ASP i removed the
IIS_USER and added the newly created user with read and execute access. So
far so good, it worked for months. Now, from one day on another they can't
login anymore. Without getting an error. It just keeps popping up the
username/password dialog over and over again. The only odd thing is that,
f.e. when i type as username MyUserName and password, then when the dialog
returns in the username field there is now www.mywebsite.com\MyUserName

Strange enough, their "master" account, i.e. the account they use for
ftp-upload of the complete site, is still allowed access.

So i thought it was something with the user. I first removed his rights on
that page and put them back, but that didn't help. I then completely deleted
the user and re-created it, but that didn't help either. Then i created a
completely new user, with a completely different name, and gave him the
rights on that page. No access.

Then i made a testpage on my own website, gave those two new users
permission on it and removed the anonymous user, but i could't get access.

Then i thought: let's try that "master" account on my testpage, and when i
added it to the security settings for the page, and tried to open the page,
it wouldn't work either. So that makes me start to wonder wether it could be
something "bigger" than just a user problem. Maybe a security setting that
got changed?

So i'm kind of stuck with the fact that the pages that are already secured
still remain, but that i can't add new users to them.

The only thing that changed a.f.a.i. know are the updates that i installed
through Windows Update. But that really can't be the problem. I also thought
about a virus, but i scanned with 2 different scanners and no viruses found.

Has anyone got an idea what might be wrong?

Thx in advance!

Mr. T