Just started getting 502 Bad Gateway and in the nginx error log I see:

upstream sent invalid status "" while reading response header from upstream

When I restarted php-fpm, everything went back to normal and working. Did something with php-fpm just go wrong? I don't see anything in the php-fpm error logs. What typically causes this, and what are some fixes to prevent it from happening again.

I'm having the same problem. You are lucky because even though I restarted php-fpm, it didn't go back to normal...

I faced the same problem too, and restarted php-fpm and things went back fine.
I don't know the cause, but seems it's related to nginx