Unable to send push notification through ACS API

When I execute the following command from OS X, push notifications through ACS succeed.

curl -X POST -F "channel=test" -F "to_tokens=<device_token>" -F "payload=Test message" "https://api.cloud.appcelerator.com/v1/push_notification/notify_tokens.json?key=<APP_KEY>&pretty_json=true"

The ACS response is

{
  "meta": {
    "status": "ok",
    "code": 200,
    "method_name": "NotifyTokens"
  }

But when I run the exact same command from a linux host, it fails with

{
  "meta": {
    "status": "ok",
    "code": 200,
    "method_name": "NotifyTokens",
    "message": "Unable to send notification to <device_token>"
  }

Both messages appear in the ACS API log (https://cloud.appcelerator.com/apps/<appId>/log ) as 'successful', but in the push notification log (https://cloud.appcelerator.com/apps/<appId/push_notification/logs#) for the linux generated message I see "No sending logs found for this push notification."

OS X curl: curl 7.37.1 (x86_64-apple-darwin14.0) libcurl/7.37.1 SecureTransport zlib/1.2.5

linux (ubuntu): curl 7.35.0 (x86_64-pc-linux-gnu) libcurl/7.35.0 OpenSSL/1.0.1f zlib/1.2.8 libidn/1.28 librtmp/2.3

Any ideas why it doesn't work from the linux host?