Having a username & password specified shouldn't prevent your browser accessing Sabnzbd+. If that IS what's happening it might mean your browser is the problem and not the add-on. Did you have SSL enabled also?
I have SSL and username / password authentication enabled on my Ultra, and it works fine. Admittedly i'm running on x86, not ARM like you. But the functionality of the add-on ought to be the same across platforms.
One thing i would say though, is if you are intending to run Sabnzbd+ without authentication, make sure it's not accessible from the internet, or you might get a surprise you don't want, if you know what I mean.