If you have performed a factory default reset a few times and you continue to see the "unexpected error" message, then the AirPort is most likely defective.
The only other thing that I can think of that would cause this type of error would be an unsupported type of setup.....in other words, you are trying to get the AirPort Extreme to do something that it was not designed to do.
Normally, you would have an AirPort Extreme connected to your modem with a wired Ethernet cable connection. However, if you already have another Apple router that is providing a wireless network, the AirPort Extreme can be set up to "extend" or "repeat" that network using a wireless connection.
I can't tell whether you are getting the error message before you try to set up the AirPort Extreme....or....whether you can go through the setup, and when you try to Update the device to save the setup, the error occurs at that time.
Can you clarify and provide more details on how you are trying to set up the AirPort.....if you can get that far?