This is a weird one. I have a WSS3 site, no MOSS, with a custom membership and role provider that authenticates against CRM. All the users have also been added to the site user list so once logged in they have correct display names.

On dev and stage everything works fine, but on UAT the users can't get past the login screen. The login screen is working, in that if you type an incorrect password for a user it comes back with the right message, meaning the custom provider is working fine. If you fill the login form in correctly you are immediately redirected straight back to the login screen, with the IIS logs showing that the login screen sent the authenticated user to the site and then was sent back. Setting the site to allow anonymous access shows that the user is not logged in on the site side after authenticating correctly.

The ULS logs show: The user does not exist or is not unique.
Found 1 trusted forests nzct.local. Found 0 trusted domains

Adding logging code to the site I have verified that the membership provider is correctly set, and can find the user when asked. Also, when accessing the site user list, I can find the SP user with the right name.

It just refuses to set the current user to be the authenticated user. Weird.