Can anybody explain how authentication works with the Deployment service

We have one system where everything works perfectly, expect deployment.

The service is fine, the engine is fine. The database is fine. Users are set up. Configuration of various xml files seems to be identical to other systems that do work. We have repeatedly uninstalled and reinstalled. Created new databases by the dozen. We keep gettinb the same error with absolutely no indication of the cause:

"Invalid user name or password for repository '[none]'"

I can only assume this has to do with the net.tcp::///Deployment URL that is set up. I cannot see where or how this is set up (yes, net.tcp port sharing service is running). There is no actual documentation on this, only what to add to or change in the xml file.

Can anybody guess why the Designer would not be able to find the service, and where any configuration of the service may be made?