Thanks... that was indeed the password. The login is Sentinel.
Anyway, the problems turned out to be something else. After a reboot I could login... but after 1 hour or so I couldn't. After a few tests it turns out this is consistent... I can only login to the web interface for about an hour after a reboot.
The good news is that I only log into this thing once every couple years. So while a firmware update or something may fix it, I'm not too concerned.