Request dies silently

Hello.

I'm having a weird problem. I have a REST API built with Phalcon, and at one occasion, a business-critical request silently dies half-way through the request, triggering an empty 200 from Phalcon.

I've been able to pinpoint some specifics regarding the error. It only appears if I have binary data sent with cURL, and it reaches my Bootstrap properly (where I run Application::run(), but it doesn't reach my ACL plugin, which is next in line in my code.

Here's a cURL-request that triggers the issue:

curl -vv -X POST -f -L --data-binary "@-" -H "Content-type: application/x-www-form-urlencoded; charset=UTF-8" https://api.mysite.local/callback < callbackData.txt

Where callbackData.txt contains the following:

documentid=9222115557375343789&signedAndSealed=true

How can I troubleshoot this problem further? There's nothing in my PHP-logs.

Thanks.

EDIT FYI, there's nothing in the nginx error log, but the access log contains a proper record:

127.0.0.1 - - [27/Oct/2015:15:44:57 +0100] "POST /callback HTTP/1.1" 200 5 "-" "curl/7.43.0"