Updating API specification doesn't work

I am doing a PUT request, tried both with a JSON and YAML, I'm getting a 503 error.
The tried specifications are valid.

curl -X PUT -T "openapi/spec301.json" -u "$README_KEY:" "https://dash.readme.com/api/v1/api-specification/$SPEC_ID"

Response:

<!DOCTYPE html>
<html>

<head>
	<meta name="viewport" content="width=device-width, initial-scale=1">
	<meta charset="utf-8">
	<title>Application Error</title>
	<style media="screen">
		html,
		body,
		iframe {
			margin: 0;
			padding: 0;
		}

		html,
		body {
			height: 100%;
			overflow: hidden;
		}

		iframe {
			width: 100%;
			height: 100%;
			border: 0;
		}
	</style>
</head>

<body>
	<iframe src="//readme.com/error"></iframe>
</body>

</html>