This is exepcted if the Unity server and the desktop you are logging into the AA from are in different domains. Joys of DCOM (which the media master uses). It doesn't get use the credentials you supplied to IIS when authenticating to the initial login, it needs it's own. You wont be prompted again after that 2nd login, however... it's good for the duration.
The client team is working on a nifty trick that will "spoof" it and pass the credentials you supplied with the first login to the media master (more or less pulling a fast on on DCOM). Currently, however, this is how it works.
Are you getting this error “Installer User Interface Mode Not Supported. The installer cannot run in this UI mode. To specify the interface mode, use the -i command-line option, followed by the UI mode identifier. The value UI mode identifiers...
The below trick might come handy when you have to add a new node to a cluster but you don't have or is unsure of the security password for the publisher. This procedure has been around for ages.
1) Login into the CLI of the Publisher.