AirPort Extreme "An unexpected error occured." after factory reset

With nothing else to do today, I decided to solve the lingering connectivity problems in the back yard. I have an old AirPort Extreme that I replaced with a Time Capsule many years ago, so I thought I would put that in the garage and pull some outdoor Ethernet. The system came up immediately when I plugged it in, and as expected I got a blinking amber light. I opened AirPort Utility on my Mac and plugged the Extreme into the TC using an Ether. It asked for a password, but I had no idea what it might be, so I did a factory reset. After the reboot, the system appears as a new WiFi device in Other Wi-Fi Devices in AirPort Utility under the "New AirPort Base Stations" group, which is what I would expect. However, when I select it in the menu, after briefly saying "Joining AirPort Extreme", a large dialog box appears: "An unexpected error occured. Try again" Off to a great start! I tried another reset, as well as a power cycle, same outcome. Where to go from here?