Internal server error. The response's detail should clarify the problem.
This example shows a typical 500 error:
{
"type": "https://problems.luna.akamaiapis.net/papi/proxy/forward-request-error",
"title": "Forward request error",
"detail": "Failed to make forward request",
"instance": "/papi/v1/properties/%7B%7Bproperty_id_clone%7D%7D/activations#abc12345abcc02a7",
"status": 500
}