Everything looks perfectly fine, I can't see why it wouldn't work.
Make sure you are using the original Mambo login module.
I would probably need to do some debugging directly on your site to find out what the cause of the problem is.
I've done a small module that you can install though and then tell me what the output from it is with different users logged in. Just install the attached zip as a Mambo module and publish "Internal User Info" module and check it from the frontpage.
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