At times, like today, I do encounter http://null:8080/auth/XUI/#login/ error when trying to log in to OpenAM.
A quick workaround is to close the browser and open a new browser or go into Incognito mode.
PS: On the browser that I encountered the above issue, I did have another OpenAM session running. But it was pointing to a totally different domain. No time to think straight yet. :) Just a quick note here since I have captured the logs for further investigation.
Updated on 10th March 2016
Peter was referring to Configuration > Authentication > Core.
I tested in my environment. It worked! Thank you! Thank you!