after upgrade to 2.9 (beta 3 I beleive), I am unable to sign into web portal with the previous admin user account or any of the default login’s i’ve found on the forums.
you were probably using the db credentials previously which don’t work any longer unless you enable that ‘back-door’ solution.
However, if you don’t have an admin capable user on your system then it would not have closed that back door meaning there is probably a user configured that you can sign in with, you just may not know it 
You can type ‘amportal admin auth_none’ to take off all authentication controlled by FreePBX to get back in, go figure out who the user is and/or their credentials (or make one) and then flip the authentication back on (either in Advanced Settings or with ‘amportal admin auth_database’