unknown_agent
What happened
A local machine proves who it is with the key it received when it was paired. When that key matches no machine on your account, the machine's requests are refused with 401. A pairing you revoked on the Engine page stops working at once. Storing a result from a machine that is not yours answers 403 with the same code.
What to do
Pair the machine again with a new code from the Engine page in the web app.
Example
json
{
"error": "That machine is not paired, or its pairing was revoked.",
"code": "unknown_agent"
}