Several people have said that when they type in the correct username and password for my site via basic access authentication, they are hit with a 401 authorization error. They are simply copying and pasting in the password, so I'm pretty sure it's not due to incorrect credentials. Considering that it could be the browser settings, I tried the following to try and reproduce the error in IE9:
Setting the security level to High (this doesn't reproduce the error)
Setting the user authentication as anonymous (this reproduces the error, but without the login that they see, so it can't be this causing it)
I can't think of anything else that could cause it. Is there an issue with older browsers or some other security setting I haven't tried? Or anything else?