I got this error this morning and it took a couple of hours to fix. Ultimately, I fixed it as follows:
1) go to amazon.com>Content and Devices>Devices. Select the Echo Dot and Deregister (click on the 3 little dots by the device name.
2) unplug the Echo Dot for at least 3 seconds, then plug it back in.
3) hold down the Microphone and Volume Down buttons for 20 seconds. The light ring should go to orange, then blue, then orange again and the voice should say "Welcome to Echo Dot. Follow the directions in your Alexa app."
4) follow the directions to set up the Echo Dot from scratch.