upstream sent too big header while reading response header from upstream

[rubenarslan]

Yes, that's quite probably it! 
Then I guess it's on Jerome to weigh in, I don't know the exact reasoning
for doing so, I would have thought it would be more appropriate to hash the
request body in the cache key, but maybe that's not possibly using nginx?
OpenCPU allows for request bodies up to 500M by default, what kind of
settings would be necessary to make that (or something more reasonable like
50M) play with the buffer? I'm guessing the buffer doesn't have to be as
large as the maximal request, right?

Posted at Nginx Forum: http://forum.nginx.org/read.php?2,247230,247352#msg-247352