prompted for login and then 401.1 Access Denied when accessing Shared Services

Many people have written about this problem. When you are in the SharePoint Central Administration site, and click one of the Shared Services you created on the left Quick Launch menu (e.g. SharedServices1), you are prompted for login repeatedly and it eventually shows your the 401.1 Unauthorized: Access is denied due to invalid credentials error.

I have summarized below the possible causes and fixes that I have found on the web:

1. Because the SSP URL has a domain name different from the machine name. This is almost always the case. A security feature called "loopback check" introduced in Windows 2003 SP1 and onward is the reason fro this. Apply a registry change on the server by following the MSFT support at http://support.microsoft.com/kb/926642/
   
   
2. Because the name of the SSP you created (e.g. SharedServices1) happens to be the same as the name of the AppPool for the SharePoint Web Application that hosts the Shared Services Admin site. They must be named differently as the SSP instance will creates a AppPool automatically under its name.
   
   
3. The SSP site collection administrators has only one user account. Not sure why this is would be a problem but adding a secondary administrator to the site collection helped with some of this error.