I am getting random "upstream sent unexpected FastCGI record: 3 while reading response header from upstream" while using Nginx with PHP7. So far I was waiting for bug https://bugs.php.net/bug.php?id=67583 to be fixed by PHP which they now have but I still get this error.

Everything works fine for few days and then suddenly I start getting this error. How can I debug this in detail? Is there a way I could see or log response from PHP (FastCGI) when this error occurs?