Recently the company web server began prompting for account passwords as
users visited sites (even redirects to external sites). No account passwords
would successfully login. After some research, I decided to reset the
Anonymous Access permissions on the server Web Sites.
That 'appeared' to work, but only for a few minutes. Soon users no longer
received the password prompt dialog. Instead, they were simply met with a
standard 401.1 "Access Denied due to invalid credentials."
I tried resetting the IUSR_<server-name> account (password only though), and
then I reset the anonymous access permissions by 'Browsing' for the user
account and manually typing in the new password.
I restarted IIS and sites and redirects began working again.
But...again...some short time (minutes) later, the 401's began again. At this
point I can restart IIS and restore full site functionality. Unfortunately, I
have to do this every 10 minutes or so to keep site availability alive.
Please...'any'...help will get you on my holiday card list!

Thanks.
-Steve