It turns out I hadn’t introduced a typo in the new admin user name when I renamed it to something less obvious than “admin” (see Hardware Frustrations). When I got the unit back from the manufacturer, reset and loaded with the new firmware that allows password recovery (see Hardware Frustrations, part II), I created a temporary second admin account, made sure I could log in with it, and then renamed the original admin account to what I thought I had renamed it to originally. And sure enough, that admin account was no longer usable! Using the second admin account, I renamed it back — and it started working again! After some more testing I realized that some user names longer than 8 characters made the login name unusable. A firmware bug! Once notified, Dataprobe promptly fixed the bug and forwarded me the fixed firmware. So if you’re buying one, be sure to check it has firmware version 1.1b80 or newer (or create a second admin account and verify that you can log in with it before renaming or deleting the default “admin” login).
Dataprobe handled the situation fairly well; I’d give them a “10″ if they would have offered to pay for the overnight shipping of the unit back to them to be unlocked since the lock-out was the result of a manufacturing flaw (firmware bug), and lack of a warning in the manual for a potential of a lock-out if the admin login is lost (with the new firmware this can no longer occur since a hardware-based password reset feature is now available).