I know this is an old post but I had a very similar same problem. My problem (which may or may not be exactly the same to yours) was that I installed mailman and postorius (the UI for mailman that by default loads on localhost:8000) and I was trying to login to that. When I started from scratch and installed the Mailman Bundler, HyperKitty had installed as well and during that install is where I was prompted for login information. That login information was what I used to login at localhost:8000 (the UI for mailman).
For what it's worth I asked almost the exact same question and once I resolved it, I traced my steps and wrote them for others. You can find my post here.