When I tried to upgrade the same version of hardware and firmware to the latest firmware I get exactly the same error. Seems that firmware updgrade is not working on this version o the latest firmaware is not correct :(.
Anyone is having the same issue? Have anyway install succesfully the latest firmware?
I have tried several time the upgrade process with different browsers and in windows 8.1 and windows 7, but it does not work.

You must update firmware with a computer that's connected to the router via ethernet. If you try to do it via Wi-Fi you will get that error.

[quote]If you try to do it via Wi-Fi you will get that error. [/quote]
I confirm it.