I could not get the nodes (I have 3) to accept the latest firmware, so I tried aredn-3.19.3.0-ubnt-airrouter-factory.bin. That worked easily. I will try to update later to see if the new firmware can be uploaded under AREDN firmware control.
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The first node I tried updated to the latest by way of the Administration tab's upload.
It seemed to take much longer to update than it should, but it works.
Thank you for posting this. I was having the same issue with a "new" AirRouter HP. I didn't remember having issues with my non-HP version so I was really stumped. I tried the version of the firmware that you suggested though and it worked like a charm!