In response to your post, I just tried to enter admin on one of my lists and thought I had the same problem, but it turned out to be that I was using my user password.
The password that you use is not necessarily your use password, but one you set up when creating the list, or one you changed the password to from the mailman administration pages. Luckily, for me, after a couple of tries with password arrangements I often use, I stumbled across the correct one, and was in with no trouble.
Unfortunately, for your situation, I have not yet found a way to display that password (even logged in), though once logged in I can change it.
It may take support to obtain the password for you; I’m going to continue to explore around and see if I can find a way to recover it, but I wanted to let you know right away that the correct password to use is one that was set-up for the list, not (though I suppose it could be, if you set it up that way), one of your existing user, mail, webid, etc. passwords (in case that might help you remember what your password might have been).
If I find a way to recover an existing password, I’ll post back. Good Luck!