Hi davidgs,
I bought an old TTOG and connected it today. Because I could not get it to work I also wanted to upgrade the firmware and had the same error. While looking through the forum I found another firmware file (see The things outdoor gateway VPN , post from rikietje. Could not find the firmware initially, but it’s on the right side of the web page!)
I could then upgrade with this firmware.
Also, if you look at the documentation/installation manual for the TTOG (The Things Outdoor Gateway | The Things Stack for LoRaWAN ) it mentions the old server (router.eu.thethings.network) instead of the new one (eu1.cloud.thethings.network). Once I changed that to the new server it immediately popped up online
Hope this helps.