In case you are still stuck try doing this, I solved it like this:
Start the decoder with power, wait for it to load the image that the decoder has factory, then go to menu> settings> software management> software update, come on OK (green key) wait for the update to finish.
Then turn off the decoder, first from the remote control and then by removing the plug.
Put the fat 32 formatted key with the OpenAtv 6.4 image for Amiko Viper 4K V20, (which you can download from here: hxxp: //images.mynonpublic.com/openatv/6.4/index.php? Open = viper4kv20) unpacked (in the root of the stick there must only be the amiko folder with its subfolders); plug in and press the green button one or more times, update should appear on the display, from the TV after a few seconds you should see two bars that signal the loading of the new image, in this way it unlocks the decoder.
At the next reboot proceed to the normal configuration of the image.
A friend gave me this decoder and I was going crazy.
Then following the above, I succeeded.
Let them know if you solve ;)