Users are being returned to the login page repeatedly, despite entering credentials which are known to be valid otherwise.
There are two main reasons why this might happen:
1. The user is authenticating, but is not listed correctly via the policy to be authorized for the target resource.
2. The SiteMinder authentication scheme being used, is hosted in a different cookie domain than the target application, and a cookie provider is not in use.
Review the SiteMinder policy to assure the user is configured to be authorized, and verify the authentication scheme utilizes a login page within the same cookie domain.