It means that in your httpd.conf file, the error directive for your 401 errors has a full url stated in it. It shoud look like this:
ErrorDocument 401 /subscription_info.html
Your's probably looks like this:
ErrorDocument 401 http://www.blah.com/subscription_info.html
This is a quote from the apache's documentation:
In addition, if you use a remote URL in an ErrorDocument 401, the client will not know to prompt the user for a password since it will not receive the 401 status code. Therefore, if you use an ErrorDocument 401 directive then it must refer to a local document.
You must have a local url stated for the 401 ErrorDocument.