Resolution:
When naming your iPlanet/SUNOne webserver instance, be sure to name it according to your standard method of URL access, i.e. machine name (http://machinename) vs. fully qualified machine (http://machinename.domain.com)

If you name your webserver instance one way, but access it another way (in the example above), certain actions, like "Cancel" in the Admin GUI will redirect the client to the name configured as the default webserver instance name, and the user will be asked to log in to PB again because the webserver sees this as a new client session.