To replicate the issue, I set up PHP-FPM and NGINX and send a POST request with a big field (say foo = "a" * 400,000), and then my server replies:

402 Payment Required

And a blank page.

I cannot figure out why this is happening!

I figured out the issue, turns out it was suhosin sending the 402 error as it triggered its request's max length filter. Whoops! suhosin writes to syslog and was complaining about it too, I just didn't look there.